United StatesDepartment ofAgriculture

Forest Service

ForestProductsLaboratory

GeneralTechnicalReportF P L - G T R - 6 9

In cooperationwith theSouthern Pine Exter ior WoodMarketing Council

in the SouthSelect ion, Appl icat ions,and Finishes

Daniel L. Cassens

William C. Feist

Abstract

Wood continues to play an important role as a struc-tural material in today’s high-tech society. As lumberand in reconstituted products, wood is commonly usedfor house siding, trim, decks, fences, and countlessother exterior and interior applications. When wood isexposed to the elements, particularly sunlight andmoisture, special precautions must be taken in struc-tural design as well as in the selection and applicationof the finish. This is especially true in the South, whereexcessive moisture can quickly damage a structure anderode the finish.

This report describes the characteristics of woodfinishes and their proper application to solid andreconstituted wood products. It describes how manu-facturing and construction practices affect the surfacesof wood products, how various types of finishes interactwith the surface, and how weathering affects thefinished surfaces. Methods for selecting and applyingvarious exterior wood finishes are presented. Finally,the failure and discoloration of wood finishes arediscussed, and methods are described for preventingthese problems. The information and advice given inthis report provide a guide for obtaining maximumservice life for finished exterior wood products in theSouth.

May 1991

Cassens, Daniel L.; Feist, William C. 1991. ExteriorWood in the South: Selection, Applications, andFinishes. Gen. Tech. Rep. FPL-GTR-69. Madison, Wl:U.S. Department of Agriculture, Forest Service, ForestProducts Laboratory. 60p.

A limited number of free copies of this publication areavailable to the public from the Forest ProductsLaboratory, One Gifford Pinchot Drive, Madison, WI53705–2398. Laboratory publications are sent to morethan 1,000 libraries in the United States and elsewhere.

The Forest Products Laboratory is maintained incooperation with the University of Wisconsin.

This publication is also available from

Exterior Woodin the SouthSelection, Applications,and Finishes

Daniel L. CassensProfessor of Wood ProductsDepartment of Forestry and Natural ResourcesPurdue UniversityWest Lafayette, Indiana

and

William C. FeistProject LeaderWood Surface Chemistry and ProtectionUSDA Forest ServiceForest Products LaboratoryMadison, Wisconsin

AcknowledgmentThis report was developed with assistance from the Technology Transfer Team on Finishing Wood Exteriorsin the South.

Contents (continued)Page

iv

I n t r o d u c t i o n

The abundance and versatility of wood have generatedthe extensive use of wood products in North America.The unique characteristics of wood make it suitable formany applications. For the most part, knowledge aboutthese characteristics has been learned throughpractical experience rather than through scientificinvestigation.

As virgin timber was cut and used by early settlers inthe United States, another forest was growing. Woodcut from this second-growth, or in some cases third-growth, forest varies somewhat from the wood cut fromvirgin growth. Nevertheless, if properly used andmaintained, the younger forest can provide economicalbuilding material that, unlike many other resources, canbe renewed for future generations. Furthermore, aminimal amount of energy is consumed during themanufacturing process. Once in place, wood continuesto conserve energy, for it is a good natural insulator.

A variety of finishes can be applied to wood. Thesefinishes include clear finishes, which reveal andaccentuate the natural beauty of wood; stains, whichimpart a rustic appearance; and paint, which can beobtained in a multitude of colors.

The availability of wood species varies somewhat byregions within the United States. Climatic conditionsalso vary tremendously—hot, humid climates cause themost rapid deterioration of wood products and finishingsystems. Consequently, construction practices, finishformulations, and durabilities also vary. This report hasbeen prepared with special consideration for thesoutheastern region of the United States.

This report describes the characteristics of woodfinishes and their proper application to solid andreconstituted wood products. It describes how manu-facturing and construction practices affect the surfacesof wood products, how various types of finishes interactwith the surface, and how weathering affects thefinished surfaces. Methods for selecting and applyingvarious exterior wood finishes are presented. Finally,the failure and discoloration of wood finishes arediscussed, and methods are given for preventing theseproblems.

H e a l t h a n d E n v i r o n m e n t a lC o n s i d e r a t i o n s

Some finishing formulations used in the past andothers that are still being manufactured and used areconsidered hazardous to health and the environment.

Research on the toxicity of finishes is ongoing, andfinishing formulations are in the process of change.Therefore, it is important to seek the most currentinformation available when choosing a finish.

Lead-Based Paint

The information in this section is taken from materialprepared by the National Association of Home Builders(NAHB) and is contained in New Life for Old Dwellings,Agriculture Handbook 481, which is in press.

Lead-based paint was widely used in residential applica-tions in the United States until the early 1940s, and itsuse was continued to some extent, particularly for theexterior of dwellings, until 1976. In 1971, Congresspassed the Lead-Based Paint Poisoning Prevention Act,and in 1976, the Consumer Product Safety Commission(CPSC) issued a ruling under this Act that limited thelead content of paint used in residential dwellings, toys,and furniture to 0.06 percent.

Lead-based paint is still manufactured today forapplications not covered by the CPSC ruling, such aspaint for metal products, particularly those made ofsteel. Occasionally, such lead-based paint (for ex-ample, surplus paint from a shipyard) inadvertently getsinto retail stores and the hands of consumers. A studyconducted for the Environmental Protection Agency in1986 indicated that about 42 million U.S. homes stillcontain interior and/or exterior lead-based paint. Asrehabilitation of these homes increases, how to abatethe toxicity of lead-based paint has become the subjectof increased public and official concern.

Studies have shown that ingestion of even minuteamounts of lead can have serious effects on health,including hypertension, fetal injury, and damage to thebrain, kidneys, and red blood cells. Low levels ofingestion can also cause partial loss of hearing,impairment of mental development and IQ, growthretardation, inhibited metabolism of Vitamin D, anddisturbances in blood formation. The American Acad-emy of Pediatrics regards lead as one of the foremosttoxicological dangers to children.

Lead-based paint applied to the exterior of homesdisintegrates into chalk and powder as a result of theeffects of moisture and ultraviolet radiation. Thisextremely fine lead dust can accumulate in the soilnear the house and can ultimately enter the house.Poor quality lead-based paint used on interior surfacescan also produce dust. Lead dust can be generatedwhen coatings on surfaces are broken through aging oras a result of rehabilitation. The dust cannot becompletely removed by conventional house-cleaning methods.

Table l—Advantages and disadvantages of strategies for abating the toxicity of lead-based paint

Abatementstrategy Advantage Disadvantage

Replacement Upgrade of finish possible Replacement possibly lower in qualityPermanent solution than original componentLow risk of failure to meet Creation of hazardous wasteclearance standards Skilled labor required for installation

Encapsulation Minimal generation of dust Protection possibly limited to short termRapid installation Routine inspection required

Expert installation required for product durabilityRoutine maintenance possibly required

Restoration possible High generation of dustLow level of skill required Difficult-to-remove lead residue left on substrate

Moderate risk of failure to meet clearancestandards and to protect workers

Use of hazardous stripping agents

Offsite paint Restoration possible Difficult-to-remove lead residue left on substrateremoval Finished product generally Potential damage to product during removal

superior to that produced from site and reinstallationby onsite paint removal

Onsite paintremoval

The methods used to abate the toxicity of lead-basedpaint or to remove the paint can themselves generatelead dust. This is particularity true when unacceptablemethods and work practices are used. Poorly per-formed abatement can be worse than no abatement atall. The micron-size lead dust particles can remainairborne for substantial periods and cannot be fullyremoved by standard cleaning methods from thesurfaces on which they have settled. When working onold painted surfaces, the worker should assume thatone or more of the paint coats contain lead. Properprecautions should be taken accordingly.

Paint coats may be checked for lead content. A por-table x-ray fluorescence (XRF) analyzer is commonlyused to determine the level of lead in paint. Becausethis device has the potential for giving very inaccurateresults if used by an inexperienced person, the analysisshould be done by a qualified professional. Chemicalspot testing, using a solution of 6 to 8 percent sodiumsulfide in water, is sometimes used to screen paintedsurfaces for the presence of lead. Be certain to checkall paint coats as the eider ones are more likely to belead based. A kit for detecting lead-based paint isavailable from the Civil Engineering Laboratory, NavalConstruction Battalion Center, Port Hueneme, CA 93043.

The removal of lead-based paints can present someserious health problems. The Department of Healthand Urban Development (HUD) has taken a leading

2

role in developing guidelines for the removal of lead-based paints. At this time, HUD has approved threeapproaches to abating the toxicity of lead-based paint:

1. Covering the painted surface with wallboard, afiberglass cloth barrier, or permanently attachedwallpaper

2. Removing the paint by scraping or heat treatment3. Replacing the entire surface to which lead-based

paint has been applied

Different strategies for abating the toxicity of lead-based paint are described in Tables 1 and 2. Certainpractices are prohibited in houses owned and operatedby HUD: machine sanding without an attached high-efficiency particulate air (HEPA) vacuum filtrationapparatus, use of propane torches, containedwaterblasting, washing, or repainting.

Removal of lead-based paint by scraping or applicationof heat does not solve the problem of lead-particulatedust. Scraping should be accompanied by misting.Dry scraping is prohibited by Maryland abatementregulations. Sanding without a HEPA-filtered vacuumshould not be used as a finishing method after scrapingor any other method of toxicity abatement. The H EPAsanders are recommended for limited surface areasonly; they are most appropriate for fiat surfaces suchas door jambs and stair risers. Open abrasive blastingis also prohibited by some regulations. For iimited

Table 2--Applications for strategies for abating the toxicity of lead-based paint

Abatementstrategy Appropriate application

Replacement

Encapsulation a

Onsite paint b

removal

Offsite paint c

removal

Exterior and interior componentsSubstitution for deteriorated

componentWindows, doors, and easily

removed building components

Exterior trim, walls, and floorsInterior floors, walls,ceilings, and pipes

Balustrades

Limited surface areasSubstitution for impractical

abatement methodsMetal substrates

Restoration projects, especiallydoors, mantels, and easilyremoved trim

Metal railings

Inappropriate application

Restoration projectsMost walls, ceilings, and floors

When historic trust requirements apply

Inappropriateness of encapsulant forsubstrate and conditions

Repainting of leaded surfaces, recoveringwith contact paper/wallpaper

Large surface areas

Some stripping procedures inappropriatefor metal

a Durability essential. Seams must be sealed to prevent escape of lead dust. Safe, effective, and estheticencapsulant for interior trim components need to be developed and tested.

b Check with manufacturer about recommendations for use on various types of wood and metal substrates.c Check with stripping company for timing of work and procedures for neutralizing and washing components.

surface areas, Agriculture Handbook 481 suggests theuse of heat-based removal methods or paint strippers.

High levels of airborne lead can be produced by heatguns, and the use of respirators is essential. Somelead is likely to be volatilized at the operating tempera-tures of most heat guns. Lead fumes are released atabout 700°F. (See Appendix A for SI conversiontable.) Heat guns capable of reaching or exceedingthis temperature should not be operated in that range.

Chemical methods for removing lead-based paint mayrequire multiple applications, depending on the numberof paint coats. Caustic and solvent-based chemicalsshould not be allowed to dry on the lead-paintedsurface. If drying occurs, paint removal will not besatisfactory, and the potential for creating lead dust willbe increased.

Chemical substances used for paint removal areusually hazardous and should be used with great care.Some solvent-based chemical strippers are flammableand require ventilation. They may contain methylenechloride, which is a central nervous system depressantthat at high concentrations can cause kidney and liver

damage and is also a possible carcinogen. Supplied-air respirators should be used when working withstrippers containing this substance. If the solvent-based strippers do not contain methylene chloride,organic vapor filters must be added to respirators.Caustic chemical strippers also have a very high pH(alkaline content), which can cause severe skin andeye injuries.

WARNING: Rehabilitation projects that will requiredisturbing, removing, or demolishing portions ofthe structure that are coated with lead-based paintpose serious problems. The home dweller shouldseek information, advice, and perhaps professionalassistance for addressing these problems. ContactHUD for the latest information on the removal oflead-based paints. Debris coated with lead-basedpaint may be regarded as hazardous waste.

Volatile Organic Compounds

Volatile organic compounds (VOC) are those organicmaterials in finishes that evaporate as the finish driesor cures. These materials are regarded as air pollut-ants, and the amount that can be released for a given

3

amount of solids or coloring pigments in the paints isnow regulated in many states. Many new regulationsare currently being established. As a result of theseregulations, many current wood finishes, includingsome latex-based materials, may be reformulated.These changes could affect the serviceability ofdifferent finishes and perhaps the method in which theyare applied. At this time, little information on the long-term performance of these new finishes is available.Such information should be available in the near future.

W o o d P r o p e r t i e s a n d

F i n i s h D u r a b i l i t y

Wood is a natural biological material and as such itsproperties vary not only from one species to anotherbut within the same species. Some differences caneven be expected in boards cut from the same tree.Within a species, factors that affect wood properties,and thus finishing characteristics, are usually related togrowth rate. Growth rate in turn is determined byclimatic factors, geographic origin, genetics, tree vigor,and competition—factors over which we currently havelittle control. The natural and manufacturing character-istics of wood are important influences on finishingcharacteristics and durability. However, most of thesecharacteristics become important only as the finish coatbegins to wear.

Natural Characteristics

Density

The properties of wood that vary greatly from speciesto species are density, grain characteristics (presenceof earlywood and latewood), texture (hardwood orsoftwood), presence of compression wood, presenceand amount of heartwood or sapwood, and the pres-ence of extractives, resins, and oils. The density ofwood, or its “weight,” is one of the most importantfactors that affect finishing characteristics. Densityvaries tremendously from species to species (Table 3),and it is important because “heavy” woods shrink andswell more than do “light” woods. This dimensionalchange in lumber and, to a lesser extent, in reconsti-tuted wood products and plywood occurs as wood,particularly in exterior applications, gains or losesmoisture with changes in the relative humidity and fromperiodic wetting caused by rain and dew. Wood inheated homes tends to dry and shrink in the fall andwinter as a result of low relative humidity, then gainmoisture and swell in the spring and summer. Exces-sive dimensional change in wood constantly stresses afilm-forming finish such as paint and may result in earlyfailure of the finish.

The amount of warping and checking that occurs aswood changes dimensions and during the naturalweathering process is also directly related to wooddensity. Cupping is probably the most common form ofwarp. Cupping is the distortion of a board that causesa deviation from flatness across the width of the piece.Wide boards cup more than do narrow boards. Boardsmay also twist from one end to the other, deviatingfrom a straight line along the length of the piece. Thisform of warp is called crook. Warping is generallycaused by uneven shrinking or swelling within theboard. Furthermore, checks, or small ruptures alongthe grain of the piece, may develop from stress setupduring the drying process or from stresses caused bythe alternate shrinking and swelling that occurs duringservice. High density (heavy) woods such as southernyellow pine tend to warp and check more than do thelow density (light) woods such as redwood (Table 3).Finally, low density woods are generally easier to nail,machine, and handle than are high density woods.

Earlywood and Latewood

The presence and amount of latewood (Fig. 1 ) insoftwood (conifer) lumber affect paint durability and areclosely related to wood density. Each year, most treespecies add one growth increment or ring to theirdiameter. For most species, this ring shows twodistinct periods of growth and therefore two bands,called earlywood (springwood) and latewood (summer-wood). Latewood is denser, harder, smoother, anddarker than earlywood, and its cells have thicker wallsand smaller cavities. The wider the latewood band, thedenser the wood. New paint adheres firmly to bothearlywood and latewood. However, old paint that hasbecome brittle with age and weathering loses itsadhesion and peels from the smooth, hard surface ofthe latewood. With varnishes, initial failure occurs overthe earlywood as a result of ultraviolet radiation degra-dation of the varnish-wood interface. If the bands oflatewood are narrow enough, as in slow growth trees,the coating may bridge the latewood and remain inplace longer than it does on the wider latewood bands.Wide latewood bands are normally absent from edge-grained cedar and redwood, improving the paintabilityof these species. However, they are prominent insouthern yellow pine and Douglas-fir, two of the mostcommon species used for general construction and forthe production of plywood. On the other hand, growthrate does not seem to significantly affect the ability ofhardwoods to retain a paint coat.

Texture

Texture refers to the general coarseness of the indi-vidual wood cells and is often used in reference tohardwoods (Fig. 2). Hardwoods are primarily com-

4

posed of relatively short, small-diameter cells (fibers)and large-diameter pores (vessels); softwoods, incontrast, are composed of longer small- diameter cells(tracheids). The size and arrangement of the poresmay outweigh the factors of density and grain pattern intheir effect on paint retention. Hardwoods with largepores, such as oak and ash, are poorly adapted toordinary housepainting methods because pinholes canform in the coating over the large pores. Pinholes areunsightly and lead to early failure of the coating. Onthe other hand, yellow-poplar has a relatively uniform,fine texture free of large pores, which improves itsoverall paintability in comparison to the coarse-texturedspecies.

Compression Wood

Compression wood is formed on the lower side of lean-ing softwood trees. The part of the growth ring with

compression wood is usually wider than the rest of thering and has a high proportion of latewood. As a result,the tree develops an eccentrically shaped stem and thepith is not centered. Compression wood, especially thelatewood, is usually duller and more lifeless in appear-ance than the rest of the wood. Compression woodpresents serious problems in wood manufacturing be-cause it is much lower in strength than is normal woodof the same density. Also, it tends to shrink exces-sively in the longitudinal direction, which causes cross-grain checking.

Heartwood and Sapwood

As trees mature, most species naturally develop adarker central column of wood called heartwood. Tothe outside of the heartwood is a lighter cylinder ofwood called sapwood. The sapwood is composed oflive cells that serve to transport water and nutrients

5

Figure l—Earlywood and Iatewood bands in southernyellow pine. Because these distinct bands often lead toearly paint failure, penetrating stains are preferable forfinishing this kind of wood.

from the roots to the leaves and to provide mechanicalsupport for the tree. The heartwood, on the otherhand, serves only as support. Heartwood is formed asthe individual cells die and are impregnated withextractives, pitch, oil, and other extraneous materials.Older trees have a higher percentage of heartwood ascompared to younger trees. Some species such assouthern yellow pine have a much wider sapwood zonethan do species like the cedars and redwood.

The old-growth timber from some species, such ascypress, is notable for its natural resistance to decayand insects. The second-growth timber contains ahigher percentage of sapwood, which has no resis-

tance to decay and insects; its heartwood is not asresistant as that of old-growth timber.

Extractives, Pitch, and Oils

Depending on species, wood may contain water-soluble extractives, pitch, or oil. Each of these sub-stances has its own properties and characteristics. Al-though these substances constitute only a small per-centage of the ovendry weight of wood, they affect, tosome degree, many wood properties including color,odor, decay and insect resistance, permeability, den-sity, and hardness. The deposition of these sub-stances is generally associated with the formation ofheartwood. Without extractives, pitch, and oil, manywoods would appear essentially identical except fortheir anatomical features.

Water-soluble extractives are extraneous materials thatare naturally deposited in the lumens, or cavities, ofcells in the heartwood of both softwoods and hard-woods. They are particularly abundant in those woodscommonly used for exterior applications, such aswestern redcedar, redwood, and cypress, and are alsofound in lesser amounts in Douglas-fir and southernyellow pine where heartwood is present. The attractivecolor, good dimensional stability, and natural decayresistance of many species are due to the presence ofextractives. However, these same extractives cancause serious finishing defects both at the time of finishapplication as well as later. Because the extractivesare water soluble, they can be dissolved when freewater is present and subsequently transported to thewood surface. When this solution of extractives

Figure 2—Left, nonporous wood (softwood or conifer such as southern yellow pine); center, ring-porous(white oak); right, diffuse-porous (yellow-poplar).

6

reaches the painted surface, the water evaporates,and the extractives remain as a reddish-brown mark.This reddish-brown mark is particularly noticeable onwhite or very light-colored paints or solid-coloredstains.

Pitch in most pines and Douglas-fir can be exuded fromeither the sapwood or heartwood. Pitch is usually amixture of rosin and turpentine; this mixture is calledresin. Rosin is brittle and remains solid at most normaltemperatures. Turpentine, on the other hand, is volatileeven at relatively low temperatures. By use of theproper kiln-drying techniques, turpentine can generallybe driven from the wood, leaving behind only the solidrosin. However, for green lumber or even dried lumbermarketed for general construction, different kiln sched-ules may be used, and the turpentine remains in thewood, mixed with the rosin. The resultant resin meltsat a much lower temperature than does pure rosin, andconsequently the mixture can move to the surface. Ifthe surface is finished, the resin may exude through thecoating or cause it to discolor or blister. This problemusually develops slowly as the air located in the resinducts and cell lumens shrinks and expands as a resultof temperature changes and forces the resin outward.The most serious problems occur when wood isheated; for example, when the sun strikes the southside of a house. Once the sticky resin is on the surfaceof the wood, the turpentine evaporates, leaving beadsof hard rosin. If the wood is painted, the resin willdiffuse through the paint coats, discoloring them.

Aromatic oils are present in some woods such ascypress, teak, and the cedars (except westernredcedar) and can cause finishing problems. Theseoils are mixtures of liquids or solids that are difficult tocrystallize. They are soluble in many common finishingfilms and thus may discolor the paint or other finish.The presence of oils may also retard drying of coatings,leaving them sticky, and often cause blistering, soften-ing, wrinkling, and general disintegration of thecoatings.

Some oils have high boiling points and evaporate onlyvery slowly, even at the temperatures used to kiln-drywood. Finishing problems with pitch and oils cangenerally be reduced through the use of the correctkiln-drying schedules. However, some care must beexercised not to carry the oil reduction process too far ifan aromatic odor in the finished product is desirable.

White or light-colored paint occasionally may acquire ayellow or brown discoloration on the heartwood ofponderosa pine and the white pines. The discolorationgenerally begins immediately over the resin passages.Later, it may diffuse throughout the coating over theheartwood, leaving the sapwood area unaffected. The

color comes from substances in the resin of the heart-wood and usually occurs on wood that is damp whilebeing painted. It rarely occurs on dry wood. Becausethe discoloration fades with sunlight, it is less of aproblem with exterior woodwork.

Manufacturing Characteristics

Some characteristics of wood, such as how the boardwas sawn from the log (which determines growth ringorientation), the presence of knots and similar irregu-larities (lumber grade), and moisture content, aredetermined primarily during the manufacturing, grading,and distributing processes. These processes canaffect the finishing characteristics and durability of solidwood products.

Ring Orientation

The manner in which a board is cut from a log affectsthe orientation of the annual rings in the piece and thusits paintability. Softwood lumber is referred to as eitherfiat grained or edge grained (plainsawed or quarter-sawed in hardwoods) or a combination of the two(Fig. 3). Most standard lumber grades contain a highpercentage of flat grain. Lumber used for board andbatten siding and shiplap is frequently flat grained.Bevel siding of redwood or cedar is generally producedin a flat-grained standard grade and an edge-grainedpremium grade. Flat-grained lumber shrinks andswells more than does edge-grained lumber and alsohas wider, darker bands of iatewood. Therefore,edge-grained lumber for siding will usually hold paintbetter than does flat-grained material. Quartersawedhardwood boards hold paint better than do plainsawedboards, but the difference is relatively small comparedto the difference between quartersawed andplainsawed softwoods.

Surface Texture

Lumber may be left in its roughsawn condition or sur-faced smooth after drying. Paint will last longer onsmooth, edge-grained surfaces. Natural finishes suchas penetrating stains or preservative treatments arepreferred for roughsawn and flat-grained lumber. Thenatural finishes often accentuate the rustic look ofroughsawn lumber and allow the wood grain and sur-face texture to show through the finish. On plywood,paint will last longer on new, rough-textured surfacesthan on smooth surfaces because more paint can beapplied to the rough surface.

Knots and Other irregularities

The presence of knots and other irregularities (such asbark, splits, pitch pockets, and insect damage) affectsthe paintability of lumber and is generally a function of

7

Cross section of log

Figure 3-Effect of sawing method on ring orientation in lumber. (M 148 631)

A

Edge-grained

lumber grade. Knots are mostly exposed end-grain.End-grained wood absorbs more finish than does flat-and edge-grained lumber, and this affects the appear-ance of the paint coating. In pine, knots often contain ahigh percentage of resin, which may cause the paintover the knot to discolor. Furthermore, large knotsusually check and crack to the extent that a noticeablesplit or defect can result. Therefore, the higher gradesof lumber intended for finishing are generally preferablefor achieving maximum serviceability of a paint coat.

Moisture Content

Finally, the moisture content of the wood is a criticalfactor in determining the service life of paint.Fortunately, the moisture content of lumber can becontrolled with a little effort, but all too often this criticalfactor is forgotten during the construction and finishingprocess. The best time to paint wood is when itsaverage moisture content is about that expected toprevail during service. Ideally, wood is installed at thisaverage moisture content. The moisture content andthus the dimensions of the piece will still fluctuatesomewhat, depending on atmospheric relativehumidity, but the change will not be excessive in eitherdirection. Therefore, film-forming finishes (such aspaints) will not be stressed unnecessarily, and a longservice life can be expected.

8

The recommended moisture content for wood used inexterior applications varies substantially depending ongeographic region (Table 4). In the South, highermoisture content values than those shown in the tablehave been experienced depending on local conditions,including elevation. However, problems associatedwith the loss or gain of moisture should be minimized ifthe moisture content is between 9 and 14 percent.Most lumber is kiln dried to acceptable or a somewhathigher moisture content before shipment. Material thathas been kept dry during shipment and storage at theconstruction site should have the desired moisturecontent.

Lumber that is marketed for construction purposes inthe kiln-dried condition but is obviously wet and some-times discolored should be rejected. If the material isused, it will dry in service, but shrinkage and accompa-nying warping, twisting, and checking will likely occur.

If the moisture content of the wood exceeds 20 percentwhen it is painted, blistering and peeling are likely.Moreover, the dark water-soluble extractives in woodslike redwood and western redcedar may discolor thepaint shortly after it is applied.

Plywood, particleboard, hardboard, and other exten-sively processed wood products undergo a significant

change in hygroscopicity during manufacture. Fre-quently, the desired equilibrium moisture content (themoisture content to which a wood product will conditionitself, depending on the relative humidity of the sur-rounding air) of such materials is not known and mayvary depending on the manufacturing process. Exten-sively processed wood products should be conditionedto the average relative humidity in the area. It is notnecessarily desirable that these products be condi-tioned to the same moisture content as that of lumber.

Finishing Characteristics

The paintability of various softwoods and hardwoodsused in the southeastern part of the United States isdescribed in Table 3 in terms of the natural woodcharacteristics of density, presence of latewood, andtexture, and of manufacturing characteristics such asring orientation. This table also provides information oncupping, checking, color of heartwood, and degree offigure. Of the softwoods, redwood and westernredcedar are rated the easiest to finish and maintain(group l), whereas southern yellow pine and Douglas-firare rated difficult to finish and maintain (group IV).Redwood and cedar are low density woods and havenarrow bands of latewood, whereas southern yellowpine and Douglas-fir are higher in density and havewide bands of latewood. The best hardwoods forpainting are fine, uniform-textured (small-pored) woodswith medium to low density such as yellow-poplar,magnolia, and cottonwood. These group III hardwoodsshould perform as well as softwoods in group Ill and IVbecause the hardwoods have less tendency to split anddo not have distinct latewood bands, which can sepa-rate, at least in part, from the earlywood bands. Theresult is a raised grain or “shell-out” effect. The groupIV hardwoods can be painted using standard proce-dures, and the service life of the finish will be only ayear or so shorter than that of group Ill hardwoods.However, repainting may be difficult unless all old paintis removed first. On hardwoods, paint tends to scaleoff in rather large flakes, apparently regardless of thegrain of the wood beneath the paint. The pores of thegroup V hardwoods are so large that they are not filledand leveled off properly by ordinary house paint. Thepores consequently become the foci for early paintfailure. Therefore, the pores must be filled with wood-filler paste prior to painting.

When group IV and V hardwoods are exposed to theweather without paint or with inadequate paint protec-tion, or when water enters behind the wood, the woodhas a marked tendency to warp or cup and pull awayfrom fastenings. These hardwoods need to be nailedfirmly, although such nailing may cause the boards tosplit. Thinner boards are more likely to cup or warpfrom surface wetting and drying than thicker boards.

Table 4--Recommended average moisturecontent values at time of installation forwood used in exterior applications suchas siding and wood trim

Moisture content(percent)

IndividualGeographical area Average pieces

Most areas of United States 12 9-14Dry southwestern areas 9 7-12Warm, humid coastal areas 12 9-14

For these reasons, l/2-in. siding of heavy hardwoods isimpractical. Boards for exterior exposure should be nothinner than 3/4 in. at any point and preferably lessthan 6 in. wide.

Where group Ill and IV hardwoods are used for exteriorcoverings of buildings, thick, rough (unplaned) boards,well-nailed with corrosion-resistant nails, will give thebest performance. If the wood is left to weathernaturally, it will mildew. Mildew occurs more rapidlyand is more extensive in warm, moist climates commonin the South. For these groups of hardwoods, certainmeasures should be taken prior to finishing, andfinishes other than paint should be applied:

1. For maximum weather tightness, use full-length,vertical tongue-and-groove siding, dip-treated in awater-repellent preservative and installed bark-sideout. All lumber should be accurately milled.

2. For good performance and ease of refinishing, usesemitransparent penetrating stains. These stainswill perform better on roughsawn boards than onnew, smooth wood. Semitransparent stains cannotbe used over painted surfaces.

C o n s t r u c t i o n P r a c t i c e s

In addition to choosing the most cost-effective woodproduct and compatible finish for a particular applica-tion, it is important to follow proper construction prac-tices during installation. In this section, we will discussgood construction practices for controlling moisturecontent in the structure and for installing siding.

Control of Moisture Content

Moisture content is critical in determining finish perfor-mance both at the time of installation and during the

9

entire life of the structure. it is also critical in prevent-ing decay and insect attack. In some cases, excessivemoisture content can render a house uninhabitablebecause the resultant mold and decay fungi produceallergic reactions in the inhabitants,

The following construction features help minimizemoisture damage to the structure and thus to theoutside paint coat or finish (Fig. 4):

1.

2.

3.

4.

5.

Wide roof overhangs provide some protection fromsun and rain to at least the upper portions of thestructure. When a 4-ft-wide overhang is provided,approximately two-thirds of a conventional one-storysidewall is protected from exposure to full sunlight.

Metal flashings under shingles at roof edges preventwater from entering the roof decking and sidewalls,particularly on roofs with a low slope that are locatedin areas of high rainfall.

Metal flashings in roof valleys, junctions of roofs andwalls, along dormers and siding material changes,and around chimneys, as well as drip caps overwindow and door frames, help prevent water fromentering the house.

Adequate and properly maintained gutters andproperly hung downspouts prevent overflow andsubsequent wetting of house eaves and sides andresulting “rainwater splash” of the siding near groundlevel (Fig. 5).

Adequate insulation and ventilation of attics and

resulting high moisture contents in the remainder ofthe house.l

6. Exhaust fans should be used to remove moisturefrom high-humidity areas such as washrooms withshowers or baths and kitchen areas. Be sure thefans are vented to the outside of the house. Clothesdryers should never be vented to the inside livingquarters or to the crawlspace, basement, or attic.Plumbing should be well maintained.

7. If the house is built on a crawlspace, a clearance ofat least 18 in. between the soil and the floor joists isrequired. The ground should be covered with a 6-milpolyethylene sheet or soil cover to prevent moisturemovement from the soil upward. The crawlspaceshould be adequately ventilated and the vents keptopen except in the coldest weather.

8. Any wood used for siding, sheathing, or platesshould be at least 8 in. (above the outside groundlineunless pressure treated with a wood preservative.

9. Proper installation of vapor barriers (vapor retard-ers), where appropriate, will help minimize conden-sation in walls and ceilings.

Visible or concealed condensation in walls and ceilingscan cause problems that range from mold and paintfailure to decay, structural damage caused by decay,and loss of thermal resistance for any insulation

1 Sherwood, Gerald E. and Robert E. Stroh. wood-frame houseconstruction. Agric. Handb. 73. Washington, DC: U.S. Department

crawlspaces prevents moisture condensation and of Agriculture; 1989 (rev.). 260p.

Figure 4-Good construction practices prevent excessive buildup of moisture in wood members. Maintainingthe correct moisture contents in wood increases the service life of finishes and decreases the risk of wooddecay. (M84 0514)

10

Figure 5-Rainwater splash on entrance door andsiding. Rain water runs from the roof and wets thesiding as it hits the concrete step.

present. Vapor retarders were originally designed tokeep water vapor out of walls and roofs to prevent or atleast minimize condensation and moisture damagefrom moisture diffusion through the wall. The conceptof vapor retarders is approximately 50 years old.However, a detailed discussion is provided here sincevapor retarders have increased in importance and ourunderstanding of them has improved.

In the 1960s, researchers concluded that the amount ofwater vapor carried by air currents from air leaks couldbe much larger than the amount delivered by watervapor diffusion. Many condensation problems inbuildings are probably associated with air leaks ratherthan vapor diffusion. Effective vapor retarders preventcondensation by limiting air flow into the wall or ceiling.

Traditionally, recommendations for vapor retardershave been based on climate. Three different climatezones are recognized in the United States, based onwinter design temperatures (Fig. 6). Zone I includesareas with design temperatures below -20°F; zone II,between 0°F and -20°F; and zone Ill, >0°F, with theexception of areas with extremely warm and humidsummers.

Vapor retarders are traditionally recommended in allexterior walls in zones I and II, and in zone Ill when thewall is insulated beyond R-4. Ceiling vapor retardershave traditionally been recommended for zone I only,but they are now recommended for zone II as well.These recommendations conform with current interimguidelines from the American Society of Heating,Refrigerating, and Air Conditioning Engineers.2

Polyethylene is the most commonly used vapor re-tarder. A thickness of at least 4 mils, and preferably

2 ASHRAE Handbook, Fundamental I-P Addition, 1989. AmericanSociety of Heating, Refrigerating, and Air Conditioning Engineers,1791 Tollu Circle NE, Atlanta, GA 30329.

more than 4 mils, is recommended. Foil-backed gyp-sum board may be used as a vapor retarder instead ofpolyethylene sheets. Blanket-type insulation that hasan aluminum or polyethylene vapor retarder attached toone face will also suffice. Kraft-paper backing is lesseffective as an air barrier or vapor retarder.

The vapor retarder must be continuous, especially if italso functions as the main air barrier in the wall orceiling. Penetrations, such as electrical outlet boxes,must be carefully sealed. Vapor retarder flangesshould be stapled to the front face of the stud, not theside of the stud. Flanges should overlap. Any torn ordamaged areas should be repaired.

It is difficult to install vapor retarders in existing build-ings without removing the interior finish material oradding a new wall covering. However, certain paintsmay have some effectiveness as a vapor retarder ifapplied to the inside surface of exterior walls. Twocoats of aluminum paint containing leafing pigment plustwo coats of decorative paint are the best for sand-finished plaster. One manufacturer of household paintshas determined that two coats of its alkyd semiglossinterior paint, which has a dry film thickness of 2.4 mils,has a vapor permeance of 0.9. Latex paints based onbutadiene-styrene resins can be used as interior vaporbarrier primer paints. However, typical latex paintsother than the butadiene-styrene paints have a rela-tively high vapor permeance and will not effectivelyretard the movement of moisture through the wall. Thevapor permeance for 4-mil polyethylene is 0.08 and forkraft and asphalt building papers, about 0.3. Remem-ber, however, that any gaps in the paint film render itessentially ineffective. Maintaining indoor humidity at asafe level is especially critical when a flueless energy-efficient or electrical furnace is installed and thebuilding becomes more airtight.

In vented or unvented crawlspaces and for floors overunheated basements, blanket insulation with anattached vapor retarder is often used. The vaporretarder is placed face-up towards the warm side of the

Figure 6-Condensation zones in the United States.

11

Installation of Siding

Figure 7—Humid climates in the continental United Statesfollow the coastal belt of the Gulf of Mexico and SouthAtlantic coastal areas.

crawlspace or floor in cold winter climates. Exposedsoil in the crawlspace should be covered with a poly-ethylene vapor retarder. When floor insulation isapplied over a partially heated basement, or the houseis located in a warm, humid climate, the vapor retardermay be placed face down.

Summer air cooling for comfort in temperate climatesdoes not usually create serious vapor problems inexterior walls and ceilings. Normally, the cooled air isnot much colder than the dewpoint of the outdoor air.Therefore, the vapor retarder should be placed in thebest location for preventing winter condensation andsummer condensation should be disregarded, eventhough some condensation may occur during thesummer. The cooling system should be properlydesigned and have the capability for adequate dehu-midification of the incoming air without overcooking.Proper maintenance and operation of the air-condition-ing system is important.

Humid climates deserve special attention (Fig. 7).Where dwellings are constantly air conditioned, warmmoist air can move from the outside and condense onthe cooler inside portion of outside walls. This situationis the reverse of cold weather condensation experi-enced in northern climates, but the same principleapplies. The amount of condensation that is likely tooccur is difficult to generalize and depends on localconditions. Therefore, successful, trouble-free localpractices should be followed. In general, exteriorsurfaces of the building should be airtight and higher invapor resistance than interior surfaces, and an interiorvapor retarder is not recommended.

Vapor retarders should be used on the outside ofexterior walls. Any water that does enter the outsidesurface of the structure can flow to the inside where itcan be removed by the air-conditioning system insteadof accumulating in the floor, wall, or roof construction.

Good construction practices for installing siding aregoverned by the type of siding used. These construc-tion practices assume that the lumber and other woodproducts have been properly stored on-site. Forexample, the wood has been protected from moisture,accumulation of dirt, and abrasion. It is essential thatthe wood be kept dry before and during construction.

In the following section, we will describe specificationsfor installing siding made from lumber and plywood.The section also includes specifications for installingsiding over rigid-foam insulation.

Lumber Siding

Wood siding is relatively simple to install. It is precisionmanufactured to standard sizes in different patterns(Fig. 8) and is easily cut, fitted, and fixed in place withordinary tools.

Courses of horizontal siding should be spaced so that asingle board runs continuously above and belowwindows and doors without notching or splicing (Fig. 9).Bevel siding that is 6 in. wide should have at least 1 in.of overlap between courses. Siding that is 8 in. orwider should overlap 1 to 1-1/2 in., depending onspacing required between window heights.

Siding should be butted snugly and squarely againstdoor and window casings, corner boards, and adjoiningboards. (Corner boards should lie flat against thesheathing.) Mitered corners should also be preciselyfitted (Fig. 10). Even if metal corner covers are used,siding boards should be carefully cut to avoid leaving ahollow place in the wood joint where water couldcollect.

All nailing should be over studs, and the total effectivepenetration of the nail into the wood should be at least1-1/2 in. For example, 3/8-in. -thick siding over 3/4-in.wood sheathing requires a sevenpenny (7d) nail, whichis 2-1/4 in. long. This combination results in a l-1/8-in.penetration of the nail into the stud, and a total effectivepenetration of 1-7/8 in. into the wood. However, nailsthat are longer than necessary should be avoidedbecause they may interfere with electrical wires andplumbing.

To fasten siding in place, hot-dipped galvanized,aluminum, stainless steel, or other noncorrosive nailsare recommended. Aluminum or stainless steel nailsare best for naturally finished siding. Plain steel-wirenails, especially the large-headed type that are de-signed for flush driving, often make unsightly rust spotson most paints. Even small-headed, plain steel-wire

12

Figure 8—Different siding patterns and recommended nailing methods for various types of woodsiding. (M 139 819)

Figure 9--Installation of bevel siding. (ML83 5110)

Figure 10-Recommended procedures for cornersof siding: (A) miter corner, (B) metal corner, (C) cornerboards, (D) siding return at roof, and (E) interior corner.(M 134 776)

13

nails, countersunk and puttied, are likely to spot thefinish with rust eventually.

For best performance, nailing patterns for various kindsof siding and application procedures should complywith the recommendations of the siding manufacturers(Fig. 8). If possible, solid lumber siding should befastened so that boards are free to shrink and swell,thereby reducing the tensile stresses that develop atthe fasteners and often result in cracking and splitting.

For plain bevel patterns, the siding should be facenailed, one nail per bearing, so that the nail clears theedge of the undercourse. Eightpenny or 10d nails arerecommended for 1 -in. -thick siding and 6d (2 in.) to 8d(2-1/2 in.) nails for thinner material.

Shiplap siding in 4- and 6-in. widths should be facenailed, one nail per bearing, 1 in. from each overlap-ping edge. One additional nail should be placed in thecenter of siding boards 8 in. or more in width. Again,8d nails should be used for 1 -in. -thick siding.

Tongue-and-groove siding, 6 in. or less in width, iseither face nailed with one 8d nail per bearing or blind-nailed with one 6d finish nail through the tongue.Boards 6 in. or more in width are face nailed with two8d nails.

In board-and-batten patterns, the underboards are spaced1/2 in. apart and nailed with one 8d or 9d (2-3/4-in.) sidingnail at the center of the board. The batten strip, 1-1/2 in.wide, is nailed at the center with one 10d (3 in.) or 12d(3-1/4-in.) nail. In board-on-board or Santa Rosa sid-ing, the underboard also is nailed with one nail at thecenter of the board. The outer boards, positioned tolap the underboards by 1 in., are face nailed with two10d or 12d nails 1-1/4 in. from the edge.

Plywood and Other Sheet Siding

Exterior-grade plywood, paper-overlaid plywood, andsimilar sheet materials used for siding are usuallyapplied vertically. When used over sheathing, plywoodshould be at least 1/4 in. thick; 5/16- and 3/8-in. -thickpanels will normally provide a more even surface.When used as sheathing and siding, plywood shouldbe at least l/2-in. thick. Hardboard should be at leastl/4-in. thick, and materials such as medium-densityfiberboard should be at least 1/2-in. thick. All types ofsheet material should have joints caulked with masticunless the joints are of the interlapping or matched typeor battens are installed. Applying a strip of 15-lbasphalt felt under uncauiked joints is also a goodpractice. When two or more sheets are applied verti-cally, metal flashing should be used to protect the top

edge of the lower sheet. The edges should also betreated with a water-repellent preservative.

Plywood should be nailed at 6-in. intervals around theperimeter and 12-in. intervals in the middle. Hardboardsiding should be nailed at 4- and 8-in. intervals.Always check the manufacturer’s recommendationsbefore installing any panel product.

Installation Over Rigid Foam Insulation

If rigid-foam, gypsum, or non-nail-base fiberboardsheathing is applied under the siding, the nail lengthsmust be adjusted to account for the sheathing thick-ness. The same effective nail penetration into solidwood as that used for lumber siding is desirable.Guidelines from the National Forest Products Associa-tion address the nailing of wood-bevel siding andhardboard-lap siding over rigid-foam sheathing. Forinstalling l/2-in. wood-bevel siding over l/2-in. rigid-foam sheathing, the guidelines recommend a 9dsmooth-shank or a 7d ring-shank wood-siding nail. If 3/4-in.rigid-foam sheathing is used, the nail size should beincreased to a 10d smooth shank or 8d ring shank.When 3/4-in. wood-bevel siding is installed over l/2-in.rigid-foam sheathing, the wood-siding nail sizesrecommended are 10d smooth shank or 8d ring shank.If 3/4-in. rigid-foam sheathing is used, the nail sizeshould be increased to 12d smooth shank or 9d ringshank. For installing 7/16-in. hardboard lap siding overeither l/2-in. or 3/4-in. (rigid-foam sheathing, a 10dsmooth-shank hardboard-siding nail is recommended.

W o o d P r o d u c t s U s e d

O u t d o o r s

Three general categories of wood products are com-monly used in construction: lumber, plywood, andreconstituted wood products (Fig. 11). Each producthas unique characteristics that will affect the durabilityof any finish applied to it. In addition, any of theseproducts may be treated with wood preservatives orfire-retardant chemicals, some of which also affect thefinishing characteristics of the product.

L u m b e r

Lumber continues to be favored for exterior application.Although this use had declined for several decades, there iscurrently an increase in the use of solid wood siding, particu-larly in multi-family units and high value homes.

Bevel siding is perhaps the most popular type of sidingfor houses. Tongue-and-groove and shiplap patternsare also used (Fig. 8), especially on buildings withoutsheathing. These patterns of siding are applied

14

Figure 11—Plywood (left), particleboard, waferboard, and hardboard (right) are manufactured or reconstitutedwood products. Special precautions should be taken when painting or staining these products.

horizontally or at an angle and tend to make a structureappear lower and longer.

Vertical siding is increasingly popular. It consists oftongue-and-groove, shiplap, or square-edged boards,which are often applied with narrow strips (battens)over the joints. Vertical siding may also be applied in aSanta Rosa pattern, with the edges of the boardsoverlapping.

Vertical siding patterns may help to reduce the effectsof light and weathering. Water drains better fromvertical boards than from horizontal. Vertical siding isalso slightly more resistant to sunlight than is beveledhorizontal siding because the angle of incident sunlightis smaller and ultraviolet radiation effects are reduced.Therefore, finishes are likely to perform somewhatbetter on vertical rather than horizontal siding.

Because it is not always cost-effective or even possibleto purchase edge-grained, irregularity-free lumber forsiding or exterior trim, paying attention to the importantcharacteristics of individual boards before installingthem can substantially lengthen the life of a paint coat.A paint coat is generally renewed as it fails on thepoorest performing boards. Individual boards with a

flat-grained pattern and wide growth rings that showthe darker Iatewood should be avoided if possible(Fig.12). Dense boards and those with excessive knotsor other defects can also cause early paint failure. Thismaterial should be used only where appearance is notimportant or for nonexposed construction, providing thematerial does not have excessive strength-reducingdefects. If these boards must be painted, place them inareas that receive little or no exposure to the sun andrain, like porch roofs or soffits. Another way to preventearly paint failure is to turn the board so that the barkside is exposed for finishing. The bark side is the sidetoward which the annual growth rings are convex.

For species that do not provide a good surface forpaint, consider applying a non-film-forming penetrating-type finish, such as a water repellant, water-repellentpreservative, or a semitransparent penetrating staininstead of paint.

Plywood

Exterior plywood manufactured from southern yellowpine, Douglas-fir, and western redcedar with smoothand roughsawn surfaces is commonly available.Roughsawn plywood with vertical grooving to simulate

15

Figure 12—Paint applied to edge-grained boards(top and bottom) performs better than that appliedto flat-grained boards (middle).

board-and-batten and other patterns is specified forexterior use (often called texture 1-11 or T 1-11 at thelumberyard). Smooth-sanded plywood is not recom-mended for siding, but it is often used in soffits. Bothsmooth and roughsawn plywood will develop surfacechecks (face checks), especially when exposed tomoisture and sunlight. These surface checks can leadto early paint failure with oil or alkyd paint systems(Fig. 13). However, this problem can be avoided byusing qualify acrylic latex stain-blocking primer andtopcoat paint systems. The flat-grained pattern presentin nearly all plywood contributes to early paint failureeven more than does face checking. Therefore, ifsmooth or roughsawn plywood is to be painted, specialprecautions should be exercised. Penetrating stainsare often appropriate for smooth-sanded androughsawn exterior plywood surfaces, but the stainsmust be renewed regularly.

Plywood should never be left unfinished if it is to beexposed outdoors. The natural weathering processdegrades the thin surface veneer of most plywood fairlyquickly. Transparent finishes are also unsuitable forplywood because they do not protect the surface fromweathering unless they contain ultraviolet radiationstabilizers and water repellents.

Exterior plywood manufactured with a medium-densitypaper overlay (MDO), in comparison to either smoothor roughsawn plywood, holds paint well. The MDOplywood is not always a stock item in many lumber-yards, but it can usually be ordered.

Reconstituted Wood Products

Reconstituted wood products are made by formingsmall pieces of wood into large sheets, usually 4 by8 ft, or as required for a specialized use such asbeveled dropsiding. These products may declassified

Figure 13-Early paint failure on plywood asa result of penetration of moisture intosurface checks.

as either fiberboard or particleboard, depending uponwhether the basic component is a wood pulp or woodchips. Along with plywood, these products account formore than half the total surface area of all materialsused as exterior siding for newly constructed dwellingsand other structures in the United States.

Cardboard is a relatively heavy type of fiberboard. Itstempered or treated form, designed for outdoor expo-sure, is used extensively as siding. Hardboard is oftensold in 4- by 8-ft sheets and as a substitute for beveleddrop siding, which has traditionally been made fromsolid wood.

Particleboard is manufactured from whole wood in theform of splinters, chips, flakes, strands, or shavings.Waferboard and flakeboard are two types of particle-board made from relatively large flakes or shavings.Oriented strandboard (OSB) is a relatively new type ofparticleboard. To improve the strength properties ofthis board, the individual particles are aligned to formseveral layers throughout the board thickness, muchIike plywood. The surface layers are oriented along theIength of the board. ln fact, OSB is currently beingused in many structural applications previously re-served for plywood.

Only reconstituted wood products manufacturedspecifically for exterior use should be used. Film-forming finishes such as paints and solid-color stainswill give the most protection to these products. Somereconstituted wood products may be factory primedwith paint, with or without a topcoat. Others maybeoverlaid with a resin-treated cellulose fiber sheet(similar to MDO plywood) or with wood veneers. Theobjective is usually to improve the surface appearanceand finishing characteristics. These products arereferred to as composites and are often used in spe-cialty applications.

16

Hardboard can provide an excellent surface for anexterior finish. However, naturally occurring water-soluble wood extractives in hardboard may leachthrough the paint coat and discolor it. The petrolatumwax used as a water repellent in some hardboardproducts may cause a discoloration problem known aswax bleed, which can usually be controlled with goodpainting practices. Research has shown that two- andthree-coat systems using acrylic latex stain-blockingprimer and topcoat finishes will give good overallperformance. Alkyd (oil-based) primers can also beused. However, a water-repellent preservative pre-treatment may not be as beneficial for hardboard panelproducts as it is for solid wood.

Research has also shown that waferboard, OSB, andparticleboard may not be suitable (without specialprecautions) for applications exposed to the weather.Waferboard is prone to delamination of particles as wellas to decay when coated with nonporous (oil-based)paint systems, even when pretreated with a water-repellent preservative. However, overlaid productsmay perform suitably, assuming good constructionpractices are used.

The edges and ends of all panel products tend toabsorb water more readily than the rest of the piece.As a result, they will often swell in thickness. Theswelled edges in particleboard, OSB, waferboard, andhardboard will not completely return to their originalthickness even when dried out. Therefore, the edgesof these products must be treated with a water-repellent preservative and painted to reduce theuptake of moisture.

The choice of siding depends on where the house isbuilt, its price range, and the architectural effectdesired. The quality of siding ranges from clear,smooth, edge-grained and rough, knotty, flat-grainedlumber and plywood to wood-based panel productsmade with or without an overlay. Special finishingtreatments based on the product are required toachieve the finish life, protection, and esthetic effectdesired.

Treated Wood Products

Wood is commonly used in outdoor applications wherespecial treatments with wood preservatives are re-quired for proper protection and best service. Wood inthese situations requires protection against decay (rot),insects, and sometimes fire. Typical examples includewood in contact with the soil or exposed to the ele-ments: decks, fences, furniture, wood roofs, andmarine structures. Table 5 shows how properly appliedwood preservatives can easily extend by tenfold theservice life of nondurable woods used in soil contact.

Table 5--Expected service life for southernyellow pine posts in soil contact pressure-treated with different preservatives andexposed in southern Mississippi a

Estimatedservice

lifePreservative (years)

Acid copper chromate (ACC) 42Ammoniacal copper arsenate (ACA) 38Chromated copper arsenate (CCA) 30b

Chromated zinc chloride (CZC) 38Coal-tar creosote 38Pentachlorophenol in oil (PCP)c 33No preservative treatment 3.3

a Forest Products Laboratory field tests.b Test still in progress.c Restricted-use pesticide.

Commercial pressure processes, when applied accord-ing to the industry standards or to the latest Federalspecifications, will provide lasting protection, providedthe material is installed properly and used as intendedby the standards. With nonpressure processes suchas brushing, spraying, and even dipping, the preserva-tive normally penetrates the ends of the wood to asmall extent and only a thin layer of the outer woodsurfaces, and therefore it will not last long in severeapplications such as ground or water contact. How-ever, these surface treatments can be useful with woodproducts used in aboveground low- to medium-decayhazard areas, such as exterior millwork, fencing, andsiding.

The three main types of preservatives are (1) preser-vative oils such as coal-tar creosote, (2) organicsolvent solutions such as pentachlorophenol, and(3) waterborne preservatives such as chromatedcopper arsenate (CCA) and ammoniacal copperarsenate (ACA). Each of these preservative classeshas its own unique characteristics and applications.

The preservative oils and the organic solvent solutionsin heavy nonvolatile oil leave the wood surface oily anddark. Creosote solutions have a strong odor. Thesetypes of preservatives are generally used in commer-cial applications where severe decay, insect, or marineborer hazards exist and where a long service life isrequired. Examples include railroad ties, bridgeconstruction, poles, and pilings. Woods treated withthese preservatives are not intended for use wherehuman contact is likely.

17

Wood treated with waterborne preservatives is com-monly available at retail lumber yards. It generally hasa clean, paintable surface (especially wood treated bya CCA-type pressure treatment), which is characterizedby a greenish or brownish color. During the treatingprocess, the wood reacts with the waterborne preser-vatives to form an insoluble residue. The chromium-containing waterborne preservatives also protectagainst ultraviolet radiation degradation, an importantfactor in the weathering process. When purchasingwaterborne preservatives, be certain to ask for theconsumer information sheet, which outlines the properuse and finishing of the material, and disposal of anyresidue.

Pressure treatment of wood with preservatives shouldnot be confused with wood treated by vacuum-pressure, dipping, or brushing processes using awater-repellent preservative. Window and door trimare often treated in this way, and the same solutioncan be brushed onto wood siding. Although thesemethods do not penetrate the wood deeply, they doprovide some protection against decay inaboveground exposure and improve the paintperformance.

NOTE: The creosote, pentachlorophenol, andwaterborne arsenical wood preservatives arerestricted pesticides and can be applied only bylicensed pesticide applicators. When purchasingwood products treated with preservatives, becertain to ask for the EPA-approved ConsumerInformation Sheet which outlines proper use,handling, and disposal procedures for this mate-rial. (See Appendix B.)

Fire Retardants and Fire-Retardant

Coatings for Wood

In addition to treatment with traditional preservatives,wood can be pressure treated with fire-retardantchemicals. The action of fire retardants depends oncomplicated chemical reactions. In general, the fire-retardant chemical reacts at temperatures below theignition point of wood to produce noncombustiblegases and water vapor. These products dilute thenormal flammable gases produced during the initialstages of combustion and thereby slow the combustionprocess. At the same time, a layer of charcoal isproduced on the outside of the wood, thus insulating itagainst further heating and the release of more flam-mable gases.

Wood can be treated with several different types of fireretardants. Substantial differences exist between thoseintended for use indoors and those for exterior applica-

tions. Some retardants used for indoor applicationstake up moisture readily and thus can prevent goodadhesion of a film-forming finish to the surface. Bloom-ing, or the movement of the treating chemical to thesurface and subsequent formation of crystals, can alsooccur. Fire-retardant-treated woods for indoor applica-tions should never be used outdoors.

Generally, different chemicals are used in the treatmentof wood intended for exterior applications. Thesechemicals normally leave a dry water-resistant surfacethat can be painted following traditional methods. Fire-retardant-treated lumber is normally kiln-dried to

19 percent moisture content after treatment. In theprocess, the material darkens somewhat, and notice-able marks often result where the lumber contacts thesmall sticks (“stickers”) used to separate the lumberduring drying. These sticker marks normally do notweather away with exposure, and application of clearor lightly pigmented stain will not cover them. If afinished appearance showing natural grain and colorcharacteristics is critical, some alternatives do exist.For example, on special orders, the lumber may besurfaced after treatment and drying. A much cleaner,brighter, and smoother surface will result. Somemanufacturers use stickers between every other layerof lumber or plywood, so that one surface is left free ofsticker marks and can be exposed.

Southern yellow pine and Douglas-fir are two speciescommonly treated with fire-retardant chemicals.Because these species normally do not hold paint andsolid-color stains as well as some other species do,recommended finishing procedures should be strictlyadhered to. Western hemlock and ponderosa pine arealso treated in this manner and have somewhat betterfinishing characteristics than southern yellow pine andDouglas-fir.

Because of the variability of wood preservatives andfire retardants available and the different treatingprocesses, the manufacturers or suppliers should beconsulted in regard to finishing details. They usuallyhave specific recommendations for achieving maximumservice life from paint and other finishes. Wood treatedwith a preservative or fire retardant should be manufac-tured in strict adherence to American Wood Preservers’Association (AWPA) standards and should contain aquality stamp indicating the treatment, treating com-pany, and inspection bureau.

Fire-retardant coatings have now been developed andmay compete with some pressure-treated materials.However, these materials are awaiting code accep-tance before they can be marketed. The coatings willbe applied at industrial plants and are not intended forgeneral consumer use.

18

W e a t h e r i n g o f W o o d

Natural weathering of wood can be considered the firstmethod of wood finishing in the United States. Duringthe first century of American colonization, exteriorsurfaces were left to weather naturally. Only later werepainted surfaces used by the general populace.Recent interest in colonial traditions and furnishings aswell as the do-it-yourself trend has revived the popular-ity of naturally weathered wood and rustic finishes(Fig. 14). Some wood houses left unfinished toweather naturally have lasted for centuries (Fig. 15).

The esthetic appeal and life expectancy of wood andthe compatibility of the wood with potential finishes aregreatly affected by the weathering process. Thisprocess, which modifies the molecular structure ofwood, results from a complex combination of chemical,mechanical, biological, and light-induced changes, allof which occur simultaneously and affect one another.In general, with 2 months of exposure to sunlight, allwoods will turn yellowish or brownish, then gray.However, dark woods eventually become lighter andlight woods become darker. Subsequently, surface

checks, then cracks may develop. The grain raisesand loosens; the boards cup and warp, pulling fasten-ers loose; and the wood surface becomes friable, withfragments separating from the surface. After theweathered gray surface has developed, usually in ayear or two, further changes are very slow to develop.

Steps in Weathering

The first step in the weathering process is the develop-ment of a yellowish or brownish color on the surface oflight-colored woods (Fig. 16). In redwood, cedar, andhardwoods with dark heartwood, the wood may firstdevelop a bleached appearance before turning brown,but browning can also occur before bleaching. Thiscolor change begins on the surface as soon as thewood is exposed to the sunlight and is relativelyshallow, ranging from 2/100 to 1/10 in. in depth. Thechange results from the decomposition of Iignin bysunlight, particularly the ultraviolet radiation as well asfrom organic materials or extractives deposited in thecell lumens of certain wood species. Lignin is thecomplex chemical structure that holds the individualcells together it constitutes from 15 to 35 percent ofthe extractive-free dry weight of wood.

Figure 15---Entrance to old log cabin in Virginia, builtin the 1850s. The structure has stood without paintor any other finish for more than 140 years.

As weathering continues, a gray Iayer, 3/1,000 to1/100 in. thick, develops. This layer is composed ofloosely matted fibers of nearly pure cellulose; rain ormoisture leaches out the decomposed brown-coloredIignin and extractives. The classic silvery gray color ischaracteristic of wood exposed to the intense radiationof the sun in cooler climates with little rain or in coastalareas where salt is present in the air.

The gray color of the surface layer of weathered woodusually results from the growth of micro-organismssuch as fungi or mildew. Certain species of theseorganisms occur anywhere a sporadic supply ofmoisture is available and can produce a uniformlyweathered and gray appearance on the wood surfacewithin a year. Unfortunately, micro-organisms mayalso produce dark spores and mycelia, which canproduce the dark gray, blotchy, and unsightly appear-ance of some weathered wood (Fig. 17). This is aparticularly serious problem with unfinished wood inhumid areas of the South. All wood surfaces willeventually turn gray when exposed to sun and rain.

Rate of Weathering

Once weathered wood turns gray, additional changesin the wood occur very slowly because the processaffects only the surface of the wood. However, thewood surface slowly wears away in a process callederosion. In general, for softwoods like pines, firs, whitecedar, redwood, and spruce, about 1/4 in. of woodthickness weathers away every 100 years. Themaximum weathering rate reported is 65/100 in. per100 years for slow-grown (24 annual rings per inch)western redcedar exposed vertically facing south. Fordense hardwoods like the oaks, the weathering rate is

Figure 16---Artist’s rendition of color and surface wood changes for a typical softwood during the outdoor weathering process.

20

only about 13/100 in. per 100 years. The weatheringrate is affected by climatic conditions, the amount ofexposure, wood density, amount of earlywood andlatewood, and ring orientation as well as growth rateand, probably, Iignin and extractives content. Ingeneral, the less dense the material and the moresevere the exposure, the faster the weathering anderosion rate.

Surface Deterioration

In addition to chemical and color changes, mechanicaldamage occurs on the exposed wood surface, mostlyas a result of moisture. Water vapor is adsorbed orreleased with changes in relative humidty. Rain ordew in contact with the unprotected wood is quicklyabsorbed by the wood surface. As the moisturecontent of wood changes, swelling and shrinking occur,resulting in stresses in the surface of the wood. Mois-ture, in combination with sunlight, causes macroscopicand microscopic intercellular and intracellular cracksand checks. Face-checking as well as warping andcupping can follow, with subsequent nail loosening.Differential swelling and shrinking of earlywood andIatewood can also raise the wood grain. Cell wallbonds near the wood surface lose strength: as watercontinues to erode the softened surface, the surfacebecomes increasingly uneven and slowly erodes away(Fig.18). Hail and windblown sand or dirt also deterio-rate the wood surface.

Although the appearance of weathered wood is attrac-tive for certain architectural effects, it does take time todevelop. Moreover, the change caused by weatheringseldom occurs evenly over different parts of a building.Those portions exposed to the most sun and rainbecome weathered first-usually the lower portions ofthe building, particularly on the south side. The topportions of the building, particularly if protected by largeoverhangs, porches, or other features, weather moreslowly. For a year or two, or even longer unprotectedareas, the wood may have a mottled appearance(Fig. 17), varying from that of freshly cut lumbar to grayweathered wood. Dark brown areas from extractives inspecies such as redwood and western redcedar maypersist even longer unprotected areas. The unequaleffects of weathering are generally not acceptable,particularly in commercial buildings where the short-term appearance is critical. Where painting is beingconsidered, wood surfaces roughened from weatheringobviously provide a very poor substrate for any film-type finish. Even a few weeks of exposure for a new,clean wood surface will decrease its paintability and theIife of the paint. On the other hand, somewhat weath-ered surfaces may be beneficial for penetrating finishesbecause they allow the wood to absorb more finishsolution.

Wood should not be allowed to weather naturally in theSouth because of the warm, humid climate. Mildew issure to develop, followed by moss. In some situations,decay may even develop with time.

Wood-Based Materials

The use of plywood, hardboard, and particleboard(including waferboard) for exterior exposure is increas-ing, The weathering of plywood is directly related tothe quality and type of the veneer exposed and to theadhesives used. Small checks are produced in themanufacture of veneer to produce plywood. Exposureto the weather will enlarge these checks, therebyallowing moisture to penetrate deeply into the wood.This is called face checking. Therefore, a water-resistant adhesive must be used for plywood. If

21

surface checks allow water to enter and becometrapped, some decay can be expected in unprotected,nondurable wood species. For these species, afinishing system containing a wood preservative shouldbe used.

Plywood face veneers generally do not exceed 1/4 in.in thickness. Therefore, excessive surface erosion,particularly of light-density species such as westernredcedar and redwood, can expose the dark-coloredglueline with time. Consequently, plywood shouldalways be protected with a finish that contains apigment. The more pigment, the greater the protection(paints offer greater protection than do stains). Trans-parent finishes containing ultraviolet radiation stabiliz-ers and water repellents can also be used, but they aregenerally less effective at protecting the plywoodsurface. Such finishes are not recommended forplywood.

Unprotected hardboard and especially particleboardpresent serious weathering problems. As the outersurfaces are exposed to changes in moisture content,shrinking and swelling of the wood particles, individualfibers, or fiber bundles result. The individual particlesor fiber bundles are loosened and separate from thesurface. Deeper and deeper layers are subsequentlyaffected at an accelerated rate. As wetting occurs,springback in the wood particles occurs as a result ofcompression set during the manufacturing process.Significant loss of strength and increased swelling canresult after only 1 or 2 years of weathering. Cohesionis lost and panels may fail under mechanical load. Forbest performance, the surfaces of hardboard andparticleboard, including the edges, must be coated withhigh-quality opaque finishes such as paints or solid-color stains.

Artificial Weathering

There are alternatives to the natural weatheringprocess for solid wood. First, water repellents or water-repellent preservatives may be used to retain the brightcolor of freshly sawn lumber. This represents a“natural finish” to many consumers. Those finishescontaining effective mildewcides work best. Second,penetrating oil-based stains (available in most colorsexcept white) can be applied to protect the wood andprovide a uniform color to the structure. Anotheralternative is to apply a commercially prepared bleach-ing oil, bleaching stain, or weathering stain. The oil orstain is essentially a water-repellent finish containingsome gray pigments. To maintain a uniformly graywood surface, the bleaching oil may have to be re-newed as needed. Or, it may be allowed to wear offnaturally, leaving the wood in a more uniform, naturallyweathered condition.

In addition to using commercially available products,wood may be treated with several different chemicalsor chemical combinations to produce a weathered oraged appearance. Little quantitative information isavailable concerning these treatments, so someexperimentation will be required to obtain an accept-able color.

Artificially stressed and weathered barn boards arealso available commercially. The surface texture ofartificially weathered wood is produced by roughsawing, sand blasting, wire brushing, and planing withnotched knives or other mechanical means. Color isusually controlled by staining or chemical treatment.

Precautions for Weathering Wood

Before wood or wood-based products are left toweather naturally, two factors should be carefullyconsidered. First, wood left to weather naturally willlikely develop mildew in warm, humid climates typical inthe South. Second, wood that becomes wet, evenperiodically, can decay. This decay should not beconfused with the surface weathering process justdescribed. Wood decay is the biological deteriorationof the cellulose and/or Iignin throughout the entirethickness of the board. To help guard against decay,which may take from one to several years to develop,all structures should be built so that exposure to bothatmospheric and ground moisture is minimized andmoisture is not trapped. Furthermore, the naturallydurable heartwood of certain species such as thecedars and redwood or preservative-treated woodshould be chosen when the chance for decay is high.

Raised grain, checking, and warping are minimal withedge-grained lumber and low-density species ascompared to flat-grained wood and high-densityspecies. Warping and cupping can be minimized if thewidth of the board does not exceed eight times thethickness. Low-density defect-free softwoods tend towarp less than do the lower grades of lumber or high-density species, especially hardwoods.

Moisture-ExcludingEffectiveness of Finishes

The various dimensions of wood and wood-basedbuilding materials are constantly changing because ofchanges in moisture content, which in turn are causedby fluctuations in the atmospheric relative humidity aswell as the periodic presence of free moisture such asrain or dew. Because film-forming wood finishes likepaint will last longer on stable wood, it is sometimesdesirable to stabilize wood by treating, finishing, orcoating it before painting.

22

The protection of wood from moisture through applyinga finish or coating depends on many variables. Amongthem are the thickness of the coating film, absence ofdefects and voids in the film, type of pigment (if any),chemical composition of the vehicle or resin, volumeratio of pigment to vehicle, vapor pressure gradientacross the film, and length of exposure period. Re-gardless of the number of coatings used, the coatingcan never be entirely moisture proof; there is no way tocompletely eliminate the changing moisture content ofwood in response to changing relative humidities. Thecoating simply slows down the rate at which the woodchanges moisture content.

The effectiveness of several different types of finishesin excluding water vapor from wood has been re-ported. 3 The numeric rating given for each finish typeis a relative value based on a three-coat systemapplied on wood at 80°F and 30 percent relativehumidity, then exposed for 14 days at the sametemperature and 90 percent relative humidity. Perfectprotection, or no adsorption of water vapor, is repre-sented by 100 percent effectiveness; complete lack ofprotection (as with unfinished wood) by 0 percent. Inthis study, the most effective coating was moltenparaffin wax applied by dipping (one coat), with a ratingof 95 percent for moisture vapor excluding effective-ness after 14 days. However, this is not a practicalfinish, and it can accumulate dirt and mildew. Thisfinish was included to demonstrate the difficulty ofexcluding moisture, especially in the vapor form, fromwood. Commercially available finishes and their three-coat effectiveness ratings after 14 days at 90 percentrelative humidity are as follows:

Effectiveness ratingFinish (percent)

Epoxy finish-clear 91

Epoxy paint-gloss 87

Aluminum flake-pigmented 84

urethane varnish

(oil-modified)

Aluminum paint 82

(Iinseed-phenolic-menhaden)

Enamel paint—satin 80

(soya-tung)

On the other hand, porous paints, such as latex paintsand low-luster (flat) or breather-type oil-based paintsformulated at a pigment volume concentration usuallyabove 40 percent, afford little protection againstmoisture. These paints permit rapid entry of water from

Figure 19---Paint normally fails initially around theends and edges of a board. Liberal application ofa water repellent or water-repellent preservative,especially to the end grain, can prolong paint Iifein these areas.

dew and rain unless applied over a nonporous primer.Likewise, penetrating finishes Iike linseed oil, tung oil,water repellents, and stain-type finishes, are ineffectiveagainst water vapor.

Good exterior coatings either retain their maximummoisture-excluding effectiveness for a considerabletime or lose effectiveness slowly. As long as theoriginal appearance and integrity of the coating areretained, most of the effectiveness remains. Paint thatis faded or chalked remains effective if vigorousrubbing removes the chalk and reveals a glossy film.Deep chalking, checking, or cracking indicates seriousimpairment of the moisture-excluding effectiveness ofthe finish.

The degree of protection provided by different treat-ments depends on the type of exposure. For example,water-repellent treatments may be ineffective against aIong-term exposure (2 weeks) to water vapor, butrelatively effective against free water for a short (1/2-h)exposure time.

For a coating to be effective in minimizing moisturecontent changes of the wood, it must be applied to allwood surfaces, particularly the end grain. The endgrain of wood absorbs moisture much faster than doesthe face grain, and finishes generally fail in this areafirst (Fig. 19). Coatings applied to one surface only willresult in unequal sorption of moisture, increasing thelikelihood of wood cupping. A buildup of the finish (twoand preferably three coats) is also required becausemere plugging of the wood pores is not effective. Thecell walls at the surface must likewise be fully covered.

3 Feist, William C., James K. Little, and Jill M. Wennesheimer. Themoisture excluding effectiveness of finishes on wood surfaces. Res.Pap. FPL 462. Madison, Wl: US. Department of Agriculture, ForestService, Forest Products Laboratory; 1985. 38 p.

23

The first or primer coat applied to bare wood is rarelyeffective. In those houses where moisture is movingfrom the living quarters to the outside wall becausethere is no vapor retarder, the application of moisture-excluding finishes to the outside will not prevent paintpeeling,

Types of Exterior WoodFinishes

Finishes or coatings are applied to exterior wood sur-faces for a variety of reasons. The particular reasonwill determine the type of finish selected and subse-quently the amount of protection provided to the woodsurface as well as the life expectancy for the finish.Finishes can be divided into two general categories:(1) opaque coatings, such as paints and solid-colorstains, and (2) natural finishes, such as waterrepellents, water-repellent preservatives, oils, andsemitransparent penetrating stains. Wood preserva-tives and fire-retardant coatings might also be called“finishes” in some respects. Various types of woodpreservatives and finishes for exterior wood are sum-marized in Table 6; suitability of finishing methods andexpected service life are summarized in Table 7.

Opaque Finishes

Paint

Paints are common coatings used on wood thatprovide the most protection against surface erosion byweathering and against wetting by water (Fig. 20).They are also used for esthetic purposes and toconceal certain defects. Paints contain substantialquantities of pigments, which account for the widerange of colors available. Some pigments will essen-tially eliminate ultraviolet radiation degradation of thewood surface. Oil-based or alkyd-based paints are asuspension of inorganic pigments in a resin vehicle anda petroleum or turpentine solvent that helps carry thepigment particles and the bonding agent (resin) to thewood surface. Latex paints are likewise a suspensionof inorganic pigments and various latex resins, but thesolvent or dispersant in this case is water.

Oil-based paint films usually provide the best shieldfrom liquid water and water vapor. However, they arenot necessarily the most durable because they becomebrittle over time. No matter how well sealed, wood stillmoves with seasonal humidity, thus stressing andeventually cracking the brittle paint. On the other hand,latex paints, particularly all-acrylic paint, remain moreflexible with age. Even though latex paints allow morewater vapor to pass through, they hold up better by

stretching and shrinking with the wood, Test fences atthe Forest Products Laboratory and other laboratoriesshow that all-acrylic latex topcoat paints applied in twocoats over a stain-blocking acrylic latex primer lastlonger than other paint systems even on difficult-to-paint roughsawn plywood surfaces.

Latex paints are generally easier to use than oil-basedpaints because water is used for cleanup. They arealso porous and will allow some moisture movement.In comparison, oil-based paints require organic sol-vents for cleanup, and at least some are nonporous.A nonporous paint film retards penetration of outsidemoisture and reduces the problem of discoloration bywood extractives, peeling of paint from outside mois-ture sources, and checking and warping of the wood.However, paint is not a preservative. It will notprevent decay if conditions are favorable for fungalgrowth.

Paints perform best on smooth, edge-grained lumber oflight-density species such as redwood and cedar andare the only way to achieve a bright, white finish.Paints are applied to the wood surface and do notpenetrate the wood deeply. Rather, the wood grain iscompletely obscured and a surface film is formed. Thisfilm can blister or peel if the wood is wetted or if insidewater vapor moves through the house wall and woodsiding because of the absence of a vapor barrier.Original and maintenance costs are often higher for apaint finish than for a water-repellent preservative orpenetrating stain finish (Table 6).

Most complaints about paint involve low-cost products,indicating that quality paints are usually worth the extramoney. Better quality paints usually contain 50 percentsolids by weight. Paints with a lower percentage ofsolids may cost less by the gallon but be more expen-sive per pound of solids, and more or heavier coats willhave to be applied to achieve equal coverage. TheForest Products Laboratory evaluates paints by generictype only. Consumer Reports (256 Washington St.,Mount Vernon, NY 10550) describes the results ofextensive weather testing by paint brand as do otherpublications.

Solid-Color Stains

Solid-color stains (also called hiding, heavy-bodied, oropaque stains) are opaque finishes that come in a widerange of colors and are essentially thin paints. Solid-color stains are made with a much higher concentrationof pigment than are the semitransparent penetratingstains, but a somewhat lower concentration of pigmentthan that of standard paints. As a result, solid-colorstains obscure the natural wood color and grain, and

24

Table 6--Initial application and maintenance of exterior wood finishes a

Initial application Maintenance

AppearanceFinish Process cost of wood Process cost Timing

Waterborne Brushingpreservative

Pressure

Diffusion pluspaint

Organic solvent Pressure, steep-preservativeb ing, dipping,

and brushing

Water repellent One or two brushand oilsc coats of clear

material or,preferably, dipapplication

Semitransparent One or twostain brush coats

Clear varnish Three coats(minimum)

Paint and Brushing: watersolid-color repellent,stain prime, and two

topcoats

Low

Medium

Low tomedium

Low tomedium

Low

Low tomedium

High

Mediumtohigh

Grain visible; Brush to remove Low 3-5 yearswood brown to surface dirtblack, fadesslightly withage

Grain visible; Brush to remove Nil, unless Nil, unlesswood greenish surface dirt stained, stained,or brownish, painted, painted,fades with age or or

varnished varnished

Grain and natural Clean and repaint Medium 7-10 yearscolor obscured

Grain visible;color asdesired

Grain andnatural colorvisible, be-coming darkerand roughertextured withage

Grain visible;color asdesired

Grain andnatural colorunchanged ifadequatelymaintained

Grain andnatural colorobscured

Brush down and Medium 2-3 yearsreapply or when

preferred

Clean and reapply Low to 1-3 yearsmedium or when

preferred

Clean and reapply Low to 3-6 yearsmedium or when

preferred

Clean and stain High 2 years orbleached areas; whenapply two breakdownmore coats begins

Clean and apply Medium 7-10 yearstopcoat, or re- for paint;d

move and repeat 3-7 yearsinitial treatment for solid-if damaged color stain

a This table is a compilation of data from the observations of many researchers.b Pentachlorophenol, bis(tri-n-butyltin oxide), copper naphthenate, copper-8-quinolinolate,

and similar materials.c With or without added preservatives. Addition of preservative helps control mildew growth.d If top-quality acrylic latex topcoats are used.

25

Table 7--Suitability and expected service life of finishes for exterior wood surfaces a

Paint andsolid-color stain

Water-repellent Semitransparent Expected lifepreservative and oil stain (years)d

Expected Expected Solid-Type of exterior Suit- iife b Suit- Iife c Suit- colorwood surface ability (years) ability (years) ability Paint stain

SidingCedar and redwood

Smooth (vertical grain)Roughsawn

Pine, fir, spruceSmooth (flat-grained)Rough (flat-grained)

ShinglesSawnSplit

Plywood (Douglas Fir andSouthern Pine)SandedTextured (smooth)Textured (roughsawn)Medium-density overlaye

Plywood (cedar and redwood)

Textured (smooth)Textured (roughsawn)

Hardboard, medium density f

SmoothUnfinishedPreprimed

TexturedUnfinishedPreprimed

Millwork (usually pine)Windows, shutters,doors, exterior trim

DeckingNew (smooth)Weathered (rough)

Glued-laminated membersSmoothRough

Oriented strandboard

HighHigh

HighHigh

HighHigh

LowLowLow

--

LowLowLow

. .-.

----

Highg

HighHigh

HighHigh

-.

1-22-3

1-22-3

2-31-2

1-21-22-3

--

1-21-22-3

----

.-

. .

. .

1-22-3

1-22-3

.-

ModerateHigh

LowHigh

HighHigh

ModerateModerate

High--

ModerateModerate

High

---.

----

Moderate

ModerateHigh

ModerateHigh

Low

2-45-8

2-34-7

4-84-8

2-42-44-8

--

2-42-45-8

----

.-

.-

2-3

2-33-6

3-46-8

1-3

HighModerate

ModerateModerate

Moderate.-

ModerateModerateModerateExcellent

ModerateModerateModerate

HighHigh

HighHigh

High

LowLow

ModerateModerate

Moderate

4-65-7

3-54-6

3-53-5

2-43-44-66-8

2-43-44-6

4-64-6

4-64-6

3-6

2-32-3

3-43-5

2-4

3-54-6

3-44-5

3-43-4

2-32-33-55-7

2-32-33-5

3-53-5

3-53-5

3-4

1-21-2

2-33-4

2-3

a These data were compiled from the observations of many researchers. Expected life predictions are foran average location in the continental United States; expected life will vary in extreme climates orexposure (such as desert, seashore, and deep woods).

b Development of mildew on surface indicates need for refinishing.c Smooth unweathered surfaces are generally finished with only one coat of stain. Roughsawn or

weathered surfaces, which are more adsorptive, can be finished with two coats; the second coat isapplied while the first coat is still wet.

d Expected life of two coats, one primer and one topcoat. Applying a second topcoat (three-coat job)will approximately double the life. Top-quality acrylic latex paints will have the best durability.

e Medium-density overlay is generally painted.f semitransparent stains are not suitable for hardboard. Solid-color stains (acrylic latex) will perform

like paints. Paints are preferred.g Exterior millwork, such as windows, should be factory treated according to Industry Standard 1S4-81.

Other trim should be liberally treated by brushing before painting.

26

Figure 20---Two southern houses maintained with good painting practices. (Photos courtesy of Southern ForestProducts Association.)

27

they can also be applied over old paints or solid-colorstains. However, surface texture is retained and a flat-finish appearance normally results. Like paints, solid-color stains protect wood against ultraviolet radiationdegradation. Oil-based solid-color stains form a thinfilm much like paint and consequently can also peelloose from the substrate. Film-forming, latex-basedsolid-color stains are also available. The acrylic-basedversions are generally the best of all the solid-colorstains. Solid-color stains are often used on texturedsurfaces and panel products such as hardboard andplywood. These stains are most effective when appliedin two or three coats. At least two coats are essentialon all panel products, and three coats are preferable.

Natural Finishes

In many locations throughout the United States, thereis a continuing trend toward the use of natural colorsand finishes to protect exterior wood siding and trim.Architects, builders, and owners are increasinglyspecifying a “natural look” for homes, apartments,churches, and commercial buildings. To some, anatural look means rough, gray, and weathered. Thisis nature’s “natural finish” (Figs. 14, 15). To others, atruly successful natural exterior wood finish is one thatwill retain the original, attractive appearance of thewood with the Ieast change in color and the leastmasking of wood grain and surface texture. In thiscase, the finish should inhibit the growth of mildewmicro-organisms, protect against moisture and sunlight,and not change the surface appearance or color of thewood.

The most natural appearance for wood is achievedwithout a protective finish. Unfortunately, in the normalweathering process, the appearance of unprotectedwood exposed outdoors is soon changed by theadverse effects of light, moisture, and the growth ofmicro-organisms on the surface. The original surfacebecomes rough as the grain raises and the woodchecks, and the checks sometimes grow into largecracks. The grain may loosen, and boards may cup,warp, and pull away from fasteners. Roughenedsurfaces change color rapidly, gather dirt, and oftenmildew, and they may become unsightly; the woodloses its surface coherence and becomes friable.Where salt in the atmosphere inhibits excessive mildewgrowth, natural weathering may create a changed butdesirable silver-gray appearance in the exposed wood.In dry or cold climates, a rustic, brown-to-gray patinamay result. In many humid locations, however, weath-ering is often accompanied by a surface growth of darkgray, black, or blotchy mildew that may remain unevenand unsightly until the wood has weathered for manyyears. In these climates, application of a “natural”finish is often desirable.

The natural finishes can be divided into two categories:(1) the penetrating types, such as transparent waterrepellents, water-repellent preservatives, oils, andsemitransparent and pigmented oil-based stains, and(2) the film-forming types, such as varnishes. Shellacsand lacquers, which are film-forming finishes, are notrecommended for exterior use because they are easilydamaged by moisture.

Water-Repellent Preservatives

A water-repellent preservative may be used as anatural finish (Fig. 21). The treatment reduces warpingand checking, prevents water staining at the edges andends of wood siding, and helps control mildew growth.Water-repellent preservatives contain a fungicide, asmall amount of wax as a water repellent, a resin ordrying oil, and a solvent such as turpentine, mineralspirits, or paraffinic oil. Some contain ultravioletradiation stabilizers. The wax reduces the absorptionof liquid water by the wood (Fig. 22), and the preserva-tive prevents wood from darkening (graying) by inhibit-ing the growth of mildew and decay organisms. Somewaterborne formulations are also available. Water-repellent preservatives without special additives will notprotect the wood surface from ultraviolet radiationdamage unless ultraviolet radiation stabilizers areadded to the finish. Ultraviolet radiation from the sunslowly degrades the surface of wood, releasing fibersand groups of fibers. The resulting finish will vary incolor, depending upon the wood color itself, but willusually weather to a clean, golden tan.

The initial application of a water-repellent preservativeto smooth surfaces is short lived, usually lasting 1 to2 years on smooth surfaces and 1 to 3 years onroughsawn or weathered surfaces. When a surfacestarts to show a blotchy discoloration resulting fromextractives or mildew, it should be cleaned with asolution of liquid household bleach and detergent andre-treated after drying (see section on prevention andremoval of mildew). During the first few years, a freshfinish may have to be applied every year or so. Afterthe wood has gradually weathered to a uniform tancolor, additional treatments may last 2 to 4 yearsbecause the weathered boards absorb more finish.

Water-repellent preservatives do not contain anycoloring pigments but will darken the color of the wood.Relatively small quantities of tinting colors can beadded to the water-repellent preservative solution toprovide special color effects; the mixture is thenclassified as a pigmented penetrating preservativestain. Two to six fluid ounces of tinting colors or colorin oil per gallon of treating solution are normally used.Colors that match the natural color of the wood andextractives are preferred. As with semitransparent

28

penetrating stains, the addition of pigment to the finishhelps stabilize the color and increase the durability ofthe finish because ultraviolet radiation is partially blocked.

Paintable water-repellent preservatives may also beused as a treatment for bare wood before priming andpainting or in areas where old paint has peeled,exposing bare wood, particularly around butt joints or incorners. This treatment keeps rain or dew frompenetrating the wood, especially at joints and on endgrain, thus decreasing the shrinking and swelling of thewood. As a result, less stress is placed on the paintfilm, and its service life is extended (Figs. 19 and 23).This stability is achieved by the small amount of wax orother water repellent present in the water-repellentpreservative. Providing the entire board has beentreated, the wax also decreases the capillary move-ment of water up the back side of lap or drop siding.The fungicide inhibits surface decay mold and mildew.For treating bare wood, make certain that themanufacturer’s label indicates that the water-repellentpreservative is paintable. Some products have toomuch wax or other water repellents and the paint willnot adhere adequately.

Water repellents are also available. These are simplywater-repellent treatments without preservative.However, solutions with preservatives will providebetter protection in the South because of the severity ofmoisture, decay, and mildew problems.

The composition of typical water repellents and water-repellent preservatives is given in Table 8.

Care should be exercised when purchasing waterrepellents or water-repellent preservatives. Manufac-turers’ specifications should be read carefully andfollowed completely. Any type of water-repellentpreservative can be used as a natural exterior finish byitself, but only some are paintable. Manufacturers havealso developed water-repellent preservatives specifi-cally for exterior natural finishes. In areas where decayis a serious problem or where wood will be in contactwith the ground or water, wood that has been pressuretreated with an appropriate preservative should bepurchased.

CAUTION: Water repellents and water-repellentpreservatives should always be mixed, handled,

Figure 21—Modern house with a natural water-repellent preservative finish. (Photo courtesy ofSouthern Forest Products Association.)

29

Figure 22— Wood surface brush-treated with waterrepellent (left). The treated surface resisted penatra-tion by liquid water, whereas the untreated woodsurface (right) absorbed water quickly.

and applied carefully. The safest place for mixingis outdoors. Solutions with solvents are volatileand flammable. Their vapors should not be inhaledor exposed to flame or sparks. Wear protectiveclothing on hands and arms and take care not tosplash the solution Into eyes or onto the face.Remember that water-repellent preservatives maycontain toxic materials. Read any labels carefully.

Oils

Many oil or oil-based natural wood finish formulationsare available for finishing exterior wood. The most com-mon oils are linseed and tung. However, these oils mayserve as a food source for mildew if applied to wood inthe absence of a mildewcide. The oils will also performbetter if a water repellent is included in the formulation.Alkyd resin and related resin commercial formulas arealso available. All these oil systems will protect wood,but their average lifetime may be only as long as that de-scribed for the water-repellent preservatives.

Semitransparent Penetrating Stains

The Forest Products Laboratory Natural Finish was oneof the first semitransparent penetrating stains formu-lated. It is the basis of some commercial formulationsand presents the option of preparing a finish from basicingredients.4

Since the development of the Forest Products Labora-tory Natural Finish, semitransparent penetrating stainshave grown in popularity and are available in nearly allpaint supply stores. These stains are moderately

4 Mack, John M., Don F. Laughnan, and Edward A. Mraz. ForestProducts Laboratory Natural finish. Research Note FPL-046.Madison, Wl: U.S. Department of Agriculture, Forest Service, ForestProducts Laboratory. Rev. 1975. 6p.

Figure 23---Treatment of wood with water-repellentpreservative. (A) window sash and frame treated andthen painted; (B) window sash and frame not treatedbefore painting. In both cases, wood was weatheredfor 5 years. Note the normally weathered paint andgood condition of the wood and glazing on the treatedstructure. (M 119 770, M 119 771)

pigmented water repellents or water-repellent preserva-tives. They penetrate the wood surface to a degree,are porous, and do not form a surface film like paint.Thus, they do not totally hide the wood grain and willnot trap moisture that may encourage decay (Fig. 24).As a result, the stains will not blister or peel even ifmoisture penetrates the wood. Design and construc-tion practices are not as critical for the long life of asemitransparent penetrating stain compared to that of afilm-forming finish.

Table 8--Composition of typical waterrepellents and water-repellent preservatives

Approximate composition(percent by weight)

Water-Water repellent

Ingredient repellent preservative

Preservativea 0 0.25-5Resin or drying oilb 10 10Paraffin wax 0.5-1 0.5-1Solvent (turpentine, 89 84-89

mineral spirits, orpaint thinner)

a Examples of preservatives used commercially and theirtypical concentrations (by weight) include copper-naphthenate, 2 percent as copper metal; zincnaphthenate, 2 percent as zinc metal; bis(tri-N-butyltin)oxide, 0.15 to 0.675 percent; copper-8-quinolinolate,0.15 to 0.675 percent; 3-iodo-2-propynyl butylcarbamate, 0.5 percent; N-(trichloromethylthio)phthalmide, 0.5 percent; and 0.5 percent2-(thiocyanomethylthio) benzo thiazole (TCMTB).

b Typical examples are boiled linseed oil, tung oil,exterior grade varnish resins, and alkyd resins.

Penetrating stains are oil based (or alkyd based), andsome may contain a fungicide (preservative ormildewcide), ultraviolet light stabilizer, or water repel-lent. Make certain that the manufacturer’s labelindicates that the finish will resist mildew or that theproduct contains a specific mildewcide. All fungicidesare not effective against mildew. Latex-based(waterborne) stains are also available, but they do notpenetrate the wood surface as do their oil-basedcounterparts. Newer latex formulations are beingdeveloped that may provide some penetratingcharacteristics.

The type and amount of pigments determine theappearance or hue of the stain. The pigment particleshelp to protect the wood surface from the degradingeffects of ultraviolet radiation from the sun. High-quality iron oxide pigments are very durable, and otherpigments may prove only slightly less so. Durability,however, depends largely on the amount of pigment orstain applied to the surface. Doubling the amount ofpigment in the formula will therefore improve durabilitybut will make the finish less transparent and the colormore intense.

Semitransparent penetrating stains are most effectiveon rough lumber or plywood surfaces because morefinish can be applied. They also provide satisfactory

performance on smooth wood surfaces but not smoothplywood surfaces. These stains are also an excellentfinish for weathered wood and flat-grained surfaces ofdense species, such as southern yellow pine, that donot hold paint well. Available from commercial sourcesin a variety of colors, semitransparent penetratingstains are especially popular in brown or red earthtones, which provide a “natural” or “rustic” appearanceto the wood. These stains are not usually available inwhite. Semitransparent penetrating stains are noteffective when applied over a solid-color stain or oldpaint coat.

NOTE: Semitransparent stains are not recom-mended for hardboard, waferboard, orientedstrandboard, and similar panel surfaces.

The first application of a semitransparent penetratingstain to a smoothly planed surface fully exposed to theweather generally lasts about 2 to 3 years (Table 6).When refinished after weathering, the finish will usuallylast much longer. Two coats of stain applied toroughsawn or weathered surfaces may last 6 to 8 yearsor more. In general, oil- or alkyd-basedsemitransparent stains provide at least twice and oftenthree times the service life of the unpigmented water-repellent preservatives or other transparent finishes.

In finishing the wood of high-density species such assouthern yellow pine and Douglas-fir, the surface maybe treated with a water-repellent preservative andallowed to weather for a year before staining. The firstcoat of stain will then penetrate uniformly and be moredurable because weathering has made the woodsurface more absorptive.

Semitransparent penetrating stains have also beenused successfully over other penetrating naturalfinishes (oils, water-repellent preservatives) that haveweathered. If the finish penetrates well into the previ-ously finished surface, it will appear flat. If the finishdoes not penetrate, it will dry slowly with numerousglossy areas and probably will not be as durable as it ison new wood. Old varnish, paint, and solid-color stainfilms must be completely removed before applying apenetrating stain.

Transparent Film-Forming Coatings

Clear coatings of conventional spar, urethane, ormarine varnish, which are film-forming finishes, are notgenerally recommended for exterior use on wood.Ultraviolet radiation from the sun penetrates thetransparent film and degrades the wood under it.Regardless of the number of coats applied, the finishwill eventually become brittle as a result of exposure tosunlight, develop severe cracks, and peel, often in less

31

than 2 years (Fig. 25). Photochemically degradedfibers peel from the wood along with the finish. If thefinish does not require a long service life, areas that areprotected from direct sunlight by an overhang or porchand are on the north side of a structure can be finishedwith exterior-grade varnish. In these areas, a minimumof three coats of finish is remmmended, and the woodshould be treated initially with a paintable water-repellent preservative. The use of varnish-compatiblepigmented stains and sealers as undercoats will alsocontribute to a longer life for the clear finish. In marineexposures, six coats of varnish should be applied forbest performance.

A finish that nearly forms a film has recently beendeveloped in Europe. This finish is commonly called avarnish stain. The film of varnish stain is thicker thanthat provided by a semitransparent stain, but thinnerthan that provided by a varnish. Varnish stains containa water repellent, special transparent iron oxidepigments, and mildewcides. The surface coating willslowly erode and can be refinished easier than thatprovided by a conventional varnish. Varnish stains areusually applied initially as three-coat systems.

There are two other types of film-forming transparentcoatings, but neither works well in exterior applications.Two-part polyurethane are tougher and perhaps moreultraviolet radiation resistant than other transparentfilm-forming coatings, but they are expensive, difficultto use, and usually have as short a life as conventionalvarnishes. The second type, lacquers and shellac, isnot suitable for exterior application, even as sealers orprimers, because these coatings have little resistanceto moisture. These finishes are also normally brittleand thus crack and check easily. However, specialtypigmented knot sealer primers based on shellac areavailable for specific exterior applications.

Fire-Retardant Coatings

Most conventional decorative coatings in themselveswill slightly reduce the flammability of wood productswhen applied in conventional film thicknesses. How-ever, for improved protection, commercial fire-retar-dant, paint-like coatings have been developed. Theseproducts provide varying degrees of protection. Fire-retardant coatings generally have low surface flamma-bility characteristics. The coatings form an expandedlow-density film upon exposure to fire (intumesce), thusinsulating the wood surface below from heat andretarding burning. Additional ingredients restrict theflaming of any released combustible vapors. Chemi-cals may also be present in these coatings that pro-mote decomposition of the wood surface to charcoaland water rather than to the formation of volatileflammable products.

Most fire-retardant coatings are intended for interioruse. Conventional paints can be applied over thecoatings to improve the durability of the fire retardant.

Application of ExteriorWood Finishes

The correct application of a finish or coating to a woodsurface is as important for durability and good perfor-mance as selecting the most appropriate finish. Allfinishes are either brushed, rolled, sprayed, or appliedby dipping. The application technique used, thequantity and quality of finish applied, the surfacecondition of the substrate, and the weather conditionsexisting at the time of application can substantiallyaffect the life expectancy of the finish. In the followingsection, we discuss different methods of finish applica-tion along with other important variables. For optimumperformance, manufacturers’ directions should alwaysbe read and followed.

Opaque Finishes

Paint

Proper surface preparation is absolutely essential forthe good performance of paint. Wood and wood-basedproducts should be fully protected from the weatherand wetting during and after instruction or installation.Surface contamination from dirt, oil, and other foreignsubstances must be eliminated. All paints and primersused in humid climates should contain a mildewcide.Preliminary research has shown that even a 3- to 4-week exposure of a freshly cut wood surface to theweather (especially the sun) can adversely affect theadhesion of paint to the wood. Wood surfaces shouldbe painted as soon as possible, weather permitting,

33

before or after installation. Wood that has weatheredbadly before painting will have a degraded surface thatis not good for painting, and a paint coating is morelikely to peel from the more degraded areas. Theweathered wood should be sanded and washed beforepainting.

To achieve maximum paint life, follow these steps:

1. Treat properly cleaned wood siding and trim with apaintable water-repellent preservative or waterrepellent and caulk all joints and cracks. Waterrepellents protect the wood against the entrance ofrain and dew and thus prevent swelling and shrinking.This is especially true for species like southern yellowpine. Water repellents can be applied by brushing ordipping. Lap and butt joints and the edges of panelproducts such as plywood, hardboard, and particleboard should be especially well treated since paintnormally fails in these areas first (Fig. 19). Allow atleast 2 warm, sunny days for adequate drying beforepainting the treated surface. If enough time is notallowed for the solvent to evaporate, the paint appliedover the treated wood may be slow to dry, maydiscolor, or may dry with a rough surface that resembles alligator hide. If the wood has been diptreated, allow it to dry for at least 1 week, if theweather is favorable. The small amount of wax (lessthan 1 percent) in a paintable water-repellent preservative will not prevent proper adhesion of the paint.

2. After the water-repellent preservative or water repellent has dried, prime the bare wood as soon aspossible. The primer coat is very important because itforms a base for all succeeding paint coats. Forwoods with water-soluble extractives, such as redwood and cedar, the best primers are good-quality oil-based, alkyd-based, or stain-blocking acrylic latex-based primer paints. The primer seals in or ties upthe extractives so that they will not bleed through thetopcoat. The primer should also be nonporous andthus inhibit the penetration of rain or dew into thewood surfaces, reducing the tendency of the wood toshrink and swell. A primer should be used whetherthe topcoat is an oil-based or latex-based paint. Forspecies, such as pine, that are predominately sapwood and free of extractives, a high-quality acryliclatex paint may be used as both a primer and topcoat.Enough primer should be applied to obscure thewood grain, but the primer should not be spread toothin, and the application rates recommended by themanufacturer should be followed. A primer coat thatis uniform, flexible, and of the proper thickness willdistribute the swelling stresses that develop in woodand thus prevent premature paint failure. The topcoatshould be applied as soon as the primer coat is dry—about 48 hours for oil-based paints--or as recommendedby the manufacturer. Special knot primers are available.

34

3. Apply two coats of a good-quality all-acrylic latexhouse paint over the primer. If it is not practical toapply two topcoats to the entire house, two topcoatsmust be applied to fully exposed areas on the southand west sides of the house to obtain good protection.Areas fully exposed to sun and rain are the first todeteriorate. Vinyl-acrylic, modified-acrylic, and oil-based topcoat paints can also be used. Allow the firstcoat of oil-based paint to cure for 1 to 2 days beforeapplying the second coat. In cold or damp weather,an extra day or two should be allowed between coatapplications. Coats of latex paint can usually beapplied within a few hours of each other. On thosewood surfaces best suited for painting, one coat of agood house paint (acrylic latex) over a properlyapplied primer (a conventional two-coat paint system)should last 4 to 5 years, but two topcoats over oneprimer coat can last up to 10 years (Table 7).

4. Apply 1 gal of paint per approximately 400 ft2 ofsmooth wood surface area. However, coverage canvary with different paints, surface characteristics, andapplication procedures. Research has indicated thatthe optimum thickness for the total dry paint coat(primer and two topcoats) is 3.5 to 5 mils or about thethickness of a sheet of newspaper. Some paints(especially latex) will successfully cover the primercoat at one-half this thickness, but these thin coatswill erode rapidly. On the other hand, thick paintcoats tend to build up and develop cracks. Thecoverage of a paint coat can be checked by applyinga pint of paint evenly over a measured area thatcorresponds to that recommended by the manufacturer. Brush application is usually superior to roller,spray, or painting-pad application, especially for thefirst coat. Professional painters can usually spraypaint and obtain good performance. The quality ofpaint is usually, but not always, related to the price.

To avoid future separation between paint coats, the firsttopcoat should be applied within 2 weeks of the primerand the second coat within 2 weeks of the first. Ascertain primer paints weather, they can form a soap-likesubstance on their surface that may prevent properadhesion of new paint coats. If more than 2 weekselapse before applying another paint coat, scrub the oldsurface with water using a bristle brush or sponge. Ifnecessary, use a mild detergent to remove all dirt anddeteriorated paint, or, if mildew develops, clean thesurface with bleach. Then, rinse the surface with waterand allow it to dry before painting. Repriming may benecessary if the old primer has weathered more than2 weeks.

To avoid temperature blistering, oil-based paints shouldnot be applied on a cool surface that will be heated bythe sun within a few hours. Temperature blistering is

most common with thick paint coats of dark colorsapplied in cool weather. The blisters usually appear inthe last coat of paint and occur within a few hours to1 or 2 days after painting. They do not contain water.

Oil-based paint may be applied when the temperatureis 40°F or above. A minimum of 50°F is desirable forapplying latex-based paints. For proper curing of latexpaint films, the temperature should not drop below 50°Ffor at least 24 hours after paint application. Lowtemperatures will result in poor coalescence of thepaint film and early paint failure.

Wrinkling, fading, or loss of gloss in oil-based paintsand streaking of latex paints can be avoided by notapplying paint in the evening of cool spring and falldays when heavy dew can form before the surface ofthe paint has thoroughly dried. Serious water absorp-tion problems and major finish failure can also occurwith some latex paints when applied under theseconditions. Allow the paint to dry for at least 2 hoursbefore sunset. Likewise, do not begin painting in themorning until the dew has had time to evaporate.

Solid-Color Stains

Solid-color stains may be applied to a smooth surfaceby brush, roller, or pad application, but brush applica-tion is usually the best. These stains act much likepaint. However, they are not generally recommendedfor horizontal wood surfaces such as decks, railings,fences, and window sills. One coat of solid-color stainis adequate, but two coats will always provide betterprotection and longer service. The all-acrylic latexsolid-color stains are generally superior to all others,especially when two coats are applied. Oil-based solid-color stains are often used as the first coat overstaining-type woods such as cedar and redwood.

Unlike paint, a solid-color stain may leave lap marks.Latex-based stains are fast-drying and are more likelyto show lap marks than are oil-based stains. Toprevent lap marks, follow the procedures suggestedunder application of semitransparent penetratingstains.

Natural Finishes

Water-Repellent Preservatives

The most effective method of applying a water-repel-lent preservative is to dip the entire board into thesolution. However, brush treatment is also effective.When wood is treated in place, liberal amounts of thesolution should be applied to all lap and butt joints,edges and ends of boards, and edges of panels whereend grain occurs. Other areas especially vulnerable to

moisture, such as the bottoms of doors and windowframes, should not be overlooked. One gallon ofpreservative will cover about 250 ft2 of smooth surfaceor 100 to 150 ft2 of rough surface. When used as anatural finish, the life expectancy of preservative onnew wood is only 1 to 2 years, depending upon thewood and exposure (see Table 7). Treatments onrough surfaces are generally longer lived than those onsmooth surfaces. Repeated brush treatment to thepoint of refusal will enhance the finish durability andperformance.

Weathering of the wood surface may be beneficial forboth water-repellent preservatives and semitransparentpenetrating stains. Weathering opens up checks andcracks, thus allowing the wood to absorb and retainmore preservative or stain, so the finish is generallymore durable. However, much more finish will berequired than if the wood were not weathered.

Oils

Oils should be applied in the same way assemitransparent penetrating stains. Care should beused in handling cloths used to apply the oils.

Semitransparent Penetrating Stains

Semitransparent penetrating stains may be applied bybrush, spray, or roller. Again, brushing will usually givebetter penetration and performance. Spraying followedby back brushing is also a good method of application.These oil-based stains are generally thin and runny, soapplication can be messy. Lap marks will form if stainsare improperly applied (Fig. 26). Lap marks can beprevented by staining only a small number of boards ora panel at a time. This method prevents the front edgeof the stained area from drying out before a logicalstopping place is reached. Working in the shade isdesirable because the drying rate is slower. Onegallon will usually cover about 200 to 400 ft2 of smoothsurface and from 100 to 200 ft2 of rough or weatheredsurface.

To achieve a long life for penetrating, oil-based stain onroughsawn or weathered lumber or plywood, use twocoats and apply the second coat before the first is dry.Apply the first coat to one panel or area. Then work onanother area so that the first coat can soak into thewood for 20 to 60 minutes. Next, apply the secondcoat before the first has dried. (If the first coat driescompletely, the second coat cannot penetrate into thewood.) Finally, about an hour after applying thesecond coat, use a cloth, sponge, or dry brush, lightlywetted with stain, to remove the excess stain. Other-wise, the stain that did not penetrate the wood will forman unsightly surface film and glossy spots. Stir stain

35

Figure 26---Lap marks formed by improper application of a semitransparent penetrating stain

thoroughly during application to prevent settling andcolor change. Avoid mixing different brands or batchesof stain.

For oil-based stains, a two-coat wet system on roughwood or plywood may last as long as 8 years in certainexposures. By comparison, if only one coat is appliedon new, smooth wood, its expected life is 2 to 4 years;life expectancy is lesson woods like western redcedar.However, succeeding coats will usually last longerbecause more stain can be applied.

CAUTION: Sponges or cloths that are wet with oil-based stain are particularly susceptible to sponta-neous combustion. To prevent fires, bury thecloths, immerse in water, or seal In an airtightcontainer immediately after use.

Latex semitransparent stains do not penetrate thewood surface but are easy to apply and less likely toform lap marks. For a long life, two coats should beapplied. Apply the second coat anytime after the firsthas dried. The second coat will remain free of gloss,even on smooth wood. These stains are essentiallyvery thin paints and perform accordingly. New formula-tions are being developed that may have some pen-etrating characteristics.

36

Transparent Film-Forming Coatings

Although short lived, transparent film-forming coatingssuch as high-quality polyurethane or spar varnish areoccasionally used for exterior applications. The woodsurface should be clean, smooth, and dry beforeapplication of the coating. The wood should first betreated with a paintable water-repellent preservative asdiscussed under painting procedures. The use ofvarnish-compatible, durable, pigmented stains andsealers or undercoats will help to extend the life of thefinishing system. At least three topcoats should beapplied. However, the life expectancy of the coatingson fully exposed surfaces is only 2 years at best. Inmarine exposures, six coats of varnish should be usedfor best performance. Varnish built up in many thincoats (as many as six) with a light sanding and a freshcoat added each year will usually perform the best.

Special Applications

Although general wood-finishing procedures areapplicable to typical situations, some applicationsdeserve special mention. These include the applicationof finish to decks and porches, fences, wood roofs, logstructures, and structures in marine environments.Wood used in all of these applications is usually

exposed to particularly harsh weathering conditions.Special consideration should be given to finish selec-tion and application. Log structures also need specialconsideration because of the large amounts of endgrain exposed and the deep checking associated withlarge timbers as well as small, round logs.

Decks and Porches

Decks and porches present a particularly severeexposure for wood and finishes. Most wood membersare in a horizontal or flat position. These horizontalsurfaces, especially in decks, are often exposed to thedirect rays of the sun and tend to collect moisture, sothe weathering process is greatly accelerated. Aswetting and drying occurs, checks tend to enlargerapidly into cracks and, along with the end-grainsurfaces, tend to retain moisture. The conditions fordecay and insect attack caused by the presence ofmoisture are thereby greatly improved. Any film-forming finish is subjected to excessive stress becauseof the continuous shrinking and swelling of the woodthat results from changes in moisture content. Further-more, the finish is subjected to abrasive wear, particu-larly in high-traffic areas. Porches are usually some-what protected, so the conditions are not normally assevere as those with decks; however, the sameconditions—moisture, sun, and abrasive wear—areusually present at least periodically. Fully enclosedporches are generally not subject to weatheringproblems.

As a result of severe weathering conditions, lumberthat has been pressure treated with waterborne preser-vatives, or naturally durable wood such as redwood, isgenerally used, at least on decks. These materialsmay be left to weather naturally or finished in a numberof ways.

For fully exposed decks, a water-repellent preservativeor a semitransparent penetrating stain may provide thebest finishing solution, even on wood that has beenpressure treated with preservatives. Special formula-tions specifically made for decks are available. Thesepenetrating deck finishes, at least the water-repellentpreservatives, may be shorter lived than paint, but theyare more easily renewed. For severe exposures, thefinish should be renewed annually; spring is usually thebest time. Light-colored penetrating stains will also lastlonger than dark ones on flat surfaces subject to trafficbecause light stains show the least contrast in graincolor as wear occurs. The penetrating finishes need tobe refinished every 1 to 2 years. To refinish, a simplecleaning of the wood with a bristle brush is usually ad-equate before applying the water-repellent or penetrat-ing finish. Paint and solid-color stains, particularly inthese applications, are likely to peel. Laborious scrap-

ing and sanding before refinishing will usually be re-quired for such finishes. Thus, paint and solid-colorstains are not appropriate for fully exposed decks.

The bright color of the wood on weathered decks canbe restored by application of commercial products(called deck cleaners, brighteners, or restorers). Theseproducts may remove the weathered wood surface andsome care should be exercised not to remove excesswood. Color can also be restored using a liquidhousehold bleach containing 5 percent sodium hypo-chlorite. The bleach is usually diluted with water (1 partbleach, 3 parts water) before it is applied to the deck.The bleach solution should be rinsed from the deckwith water. If the deck is to be finished after cleaning,allow 1 or 2 days drying time.

Paint may be used successfully on roof-protectedporch floors. The best procedure is to treat the woodwith a paintable water-repellent preservative. After thefloor dries, a primer and topcoats of porch and deckenamel should be applied. Porch enamel is especiallyformulated to resist abrasion and wear. Because oftheir low resin content, solid-color stains should neverbe used on flat surfaces such as decks and porches.

Fences

Like decks, fences are fully exposed to the weatherand have at least some parts in soil contact. As aresult, wood decay and termite attack are potentialproblems. Care must be exercised in selecting andapplying a finish to obtain a reasonable service life.This is especially true in the South.

For maximum service life, all wood fence posts usedin the South must be pressure treated with a woodpreservative. Although boards and rails may be ofnaturally durable species, pressure-treated wood willlast longer because of the severity of exposure and thevariability of naturally decay-resistant species. Bothtreated and untreated wood should be treated with awater repellent or water-repellent preservative. Besure to liberally treat the ends and the places wherepieces of wood are in contact.

Hot-dipped galvanized, stainless steel, or aluminumnails should be used wherever possible. Some alumi-num nails are soft and will bend when used on densewoods such as southern yellow pine. These types ofnails will prevent rust staining and also the possibleformation of blue-black discoloration should a water-repellent preservative or semitransparent penetratingstain be used.

Many fences are left to weather naturally. However, ifa finish is desired, semitransparent penetrating stains

37

Figure 27---Commercial structure with wood shingle roof. (Photo courtesv of Southern ForestProducts Association.)

or water-repellent preservatives that contain amildewcide or are otherwise resistant to mildew arepreferred. These finishes soak into the wood withoutforming a film, and thus they do not crack or peel.Stains come in a variety of colors and show the woodgrain. If paint is to be used, brush the surface and allends and joints liberally with a paintable water-repellent preservative and let the surface dry for atleast 2 warm, sunny days before painting. Use onecoat of a good-quality stain-blocking acrylic latexprimer, followed by two topcoats of a good-qualityacrylic latex exterior house paint. When repainting,scrape all loose paint from the wood, then use a stiffbristle brush to remove any remaining loose paint anddirt. Next, brush on a paintable water-repellentpreservative. Apply it liberally to ands of boards orpickets and to all joints. Let the treated wood dry andthen paint with acrylic latex paint.

Varnish finishes and solid-color stains are not recom-mended for exterior fences because they will notwithstand sun and rain and will require frequentrefinishing.

38

Roofs

Although wood shingles and shakes have been largelyreplaced on standard buildings by composition orasphalt-based shingles, their use is still widespread oncommercial structures and houses (Figs. 27 and 28).Shingles are sawed from large blocks of wood, aretapered from one end to the other, and generally havea relatively smooth surface. Shakes are split fromlarger blocks of wood and thus have a more ruggedappearance. Shakes may be approximately the samethickness on both ends, or they may be sawed fromcorner to corner, thus providing taper as well as onerelatively smooth side, which is turned down duringinstallation. Shakes may also have a grooved appear-ance. Shingles and shakes are used on sidewalls aswell as roofs and may be preservative or fire-retardanttreated.

Wood properties. Wood used in the manufacture ofshingles should have the following properties: (1) dura-bility, (2) freedom from splitting during nailing, (3) dimen-sional stability, that is, low ratio of tangential to radial

Figure 28—Log house with wood shake roof.

shrinkage and minimum shrinkage in all planes,(4) light weight, (5) good insulating properties,(6) adequate strength, (7) straight, even grain for easeof manufacture, (8) ability to take stains, (9) ability toresist abrasion, and (10) pleasing appearance.Edge-grained shingles will perform much better thanflat-grained. Western redcedar, redwood, cypress,and northern and southern white cedars all possessthe desired properties, but nearly all commercialshingles are currently produced from westernredcedar.

Of all wood properties to be considered, durability isprobably the most important. The heartwood of old-growth western redcedar is rated as extremely durable.The generally small amount of sapwood associatedwith this species is nondurable. There is a generalconsensus that some second-growth timber, even ofthe decay-resistant species, is not as durable as theold growth. The durability of the wood will also bedecreased as rain or other sources of moisture leachthe extractives from the wood. The heartwood extrac-tives provide cedar with natural decay resistance. Theaverage service life of a cedar shingle roof in the Southis estimated to be about 10 to 15 years, whereas ashake roof will last an average of 15 to 20 years.

In addition to western redcedar, southern yellow pinetaper-sawn shingles may be available. Research hasshown that this species can be manufactured into woodshingles and pressure treated with a wood preserva-tive. A roof free of decay for 25 to 30 years shouldresult. Other species may also be used for shingles ifproperly treated or naturally durable.

Application of shingles and shakes. Regardless ofthe type of finish, if any, proper application of shinglesand shakes is required if a long life is to be expected.Because roofs are directly exposed to moisture andrain, use only the top grade of shingles or shakesmanufactured with edge-grained heartwood (or treatedsapwood). Lower grades can be used on sidewalls orwhere an undercourse is required. Some factors toconsider when applying shingles are (1) the decking towhich the shingles are nailed, (2) the number, location,and type of nails, (3) the head lap or amount of lap overthe course below (which often depends on shinglegrade and application), (4) the space between theedges of the shingles or shakes, and (5) the separationof edge joints from one course to another. The RedCedar Shingle and Handsplit Shake Bureau (515 116thAvenue NE, Suite 275, Bellevue, WA 98004) providesan excellent publication containing details on shingle

39

and shake grades and application procedures. Similarinformation on CCA-treated southern pine shakes isavailable from the Southern Forest Products Associa-tion, P.O. Box 52468, New Orleans, LA 70152.

Application of finish. Shingles and shakes, whetheron a roof or sidewall, are often left to weather naturallyif they are made from a durable species such as west-ern redcedar. Depending on exposure and climaticconditions, the wood generally turns silver, dark gray,or dark brown. However, in warm, humid climatescommon to the South and on heavily shaded roofs andsidewalls, mildew, moss, algae, lichens, and evenwood decay can occur with time. For these reasonsand for esthetic effects, various finishes and preserva-tives can be applied to wood shingles and shakes toobtain a particular color. The types of finishes used onsidewalls may differ substantially from those used onroofs.

Weather will rapidly deteriorate any finishing system.In addition, construction methods contribute to theconditions that encourage finish and wood deteriora-tion. Because large quantities of end grain are ex-posed on shingles and shakes, the wood rapidlyabsorbs moisture, resulting in excessive shrinking andswelling. Because moisture tends to migrate from theliving quarters to the outside of a house, shingles andshakes can also absorb moisture from their back side.Furthermore, when film-forming finishes are applied toin-place roofs, small dams of finish form across thebottom of the gaps between shingles, which adds tothe amount of water absorbed by the back of theshingle. Adequate attic ventilation and sidewall vaporbarriers should be provided.

As a result of these potential problems, film-formingfinishes, such as paint, solid-color stains, and varnishor other clear, film-forming finishes should never beused on roofs. A transparent finish such as varnish willdeteriorate within a few months, while a pigmentedfinish such as paint will usually last only a few years.The result will be an unsightly appearance that isdifficult to refinish. Increased wood decay is also likelybecause the film-forming finish can help to retainmoisture in the shingle or shake. Paint and solid-colorstains can sometimes be used on shake or shinglesidewalls with better results.

If roofs or sidewalls are to be finished, semitransparentpenetrating oil-based stains provide the best perfor-mance. These stains provide color without entirelyconcealing the grain and texture of the wood. They arerelatively long lived and easily renewed, and may lastseveral years on roofs and considerably longer onsidewalls. Rough-textured edge-grained surfaces givelonger service life than do smooth surfaces. The

semitransparent stain should contain a wood preserva-tive and a mildewcide. Some paints and stains arespecially formulated for use on shingles. Those withthe highest concentration of pigment will likely give thelongest service life. Water-repellent preservatives mayalso be used on roofs and sidewalls, although their lifeexpectancy on sidewalls is only 2 to 3 years; lifeexpectancy is even less on roofs unless the treatmentsare specially formulated. These finishes contain a waxor other water repellent, a preservative, and a solventor carrier.

The first coat of finish is best applied to shingles beforethey are installed so that the backs and butts as well asthe faces are thoroughly coated. The finish may beapplied by dipping the shingles to at least two-thirdstheir length and then standing them vertically until thefinish has dried. In addition to dipping, the finish maybe applied by brushing, rolling, or spraying. Dipping isthe most effective method, followed by brushing. If thebacks are not finished, seepage of rainwater under theshingles may cause more curling than would otherwisetake place. If a light-colored finish has been applied,the butts and edges of the shingles may be discoloredby water-soluble extractives from the wood. Anyadditional coats may be applied by brushing or spray-ing after the shingles have been installed. Care shouldbe taken to coat the exposed butt ends with the finishthoroughly, preferably by brushing.

Preservatives and fire retardants. In some cases,particularly where warm temperatures and humidconditions persist for substantial parts of the year, it isdesirable to extend the life of wood shingles andshakes with special preservative treatments or fireretardants. For maximum effectiveness and long life,shingles and shakes pressure treated with thesechemicals should be purchased. Chromated copperarsenate (CCA, Type C), copper-8-quinolinolate, andcopper naphthenate are all effective wood preserva-tives when properly applied.

Maintenance. Leaves and other debris that oftenaccumulate on roofs, particularly in roof valleys andgutters, will trap moisture in shingles, increasing thelikelihood of decay. Therefore, loose debris should beroutinely cleaned from roofs and gutters. Overhanginglimbs and vines that provide excessive shade shouldalso be removed.

The roof should be checked for moss or lichen growthand be chemically treated if necessary. One simplemethod to prevent moss from developing on roofs is touse zinc, galvanized, or copper flashings. The normalcorrosion from these metals, when stretched along thebutt end of the shingles, will provide some control ofmoss (plus mold and mildew) for about 10 to 15 ft

40

downslope from the metal. Treating the roof with se-lected chemicals can also provide some protection. Asolution of copper naphthenate with 3 to 4 percentmetal content, copper octoate with 1 to 2 percent metalcontent, copper-8-quinolinate with about 1 percent ac-tive ingredient content, or other preservatives can beused to control moss, lichens, and surface decay andto prevent their growth for some time. Solutions arebest applied by brushing or dipping. Commercial for-mulations for roof treatment are available.

NOTE Manufacturers’ application end safetyrecommendations should be followed becausewood preservatives can be toxic to plants and tohumans if used improperly. Humans, animals, andvegetation should be protected from drippings andrunoff from the roof or gutters.

Regardless of the method used, surface treatments aretimited to the surface of shingles. They will not preventserious decay problems within the shingle or unex-posed parts of the roof that are not treated. However,in some cases, surface treatments can help to lengthenthe life of a wood roof by preventing the growth of mossand lichens, which retain moisture in the wood andthereby promote wood decay.

Log Structures

Interest in the construction of new log structure (Fig. 28)es well as maintenance and restoration of old ones hasincreased dramatically in recent years. These struc-tures present some unique problems and opportunitiesfor maintenance and finishing. However, before afinish is even considered, the structure must be care-fully built to keep the outside wood surface as dryas possible. Good construction practices include(1) adequate roof overhang with properly hung andmaintained gutters and downspouts; (2) roof vents, ifattic space is present (3) good drainage and ventilationaround the foundation; (4) proper venting of showers,baths, and dryers; (5) adequate clearance between thesoil and lower logs; and (6) most important of all,proper log design so that any moisture will run downand off the log rather than become trapped. Trappedmoisture is certain to result in wood decay. Oldstructures that are still sound were usually constructedwith attention to these moisture-proofing details. Also,naturally durable woods such as white oak, walnut,heartwood of white pine, and cedar were usually used,particularly for the lower courses of logs. However,even these woods will succumb to decay with pro-longed exposure to moisture.

Log houses are particularly susceptible to decaybecause of the deep seasoning checks that occur onthe surface of large wood members. These checks

Figure 29---Deep checks on an old log structureretain moisture and encourage decay.

allow moisture to penetrate into the wood member, anddecay results. Excessive end grain is also exposed,particularly on the corners of log structures (Fig. 29).This end grain as well as the notching of the logsallows for easy penetration of moisture and subsequentdecay. Proper construction, particularly wide roofoverhangs, and the application of preservative finishescan help to reduce or eliminate problems.

Because log houses are usually rustic in appearance,non-film-forming (penetrating) natural finishes arepreferable to film-forming finishes. Water-repellentpreservatives liberally applied every couple of years willallow a new wood surface to weather to light brown ortan. The preservative will also help prevent decayshould water penetrate the surface checks and cracksor end grain. However, it will not prevent decay wherepoor construction practices allow repeated wetting orcondensation. Deep surface cracks, lower courses oflogs, joints between logs, corners of the structure, andbottoms of windows and doors often trap moisture andare particularly vulnerable to decay. Consequently,these areas should be treated liberally.

Semitransparent penetrating stains may also be used ifa longer service life for the finish is preferred, if a colorother than light brown or tan is desired, and if partialcovering of the wood grain is not objectionable. Stainscontaining preservatives or mildewcides and waterrepellents are desirable for best performance.

If the structure to be finished requires chinking, applythe finish and preservative before chinking. In this way,the tops of the logs, which are particularly vulnerable towetting and subsequent decay, can be treated and thechinking will not be stained accidentally during finishapplication.

For existing structures, even those that are well weath-ered, the application of a water-repellent preservative

41

can help prevent further deterioration. Any decayedwood should be removed and the newly exposed woodliberally treated after the moisture source has beeneliminated and the wood dried. To facilitate treatment ofhard-to-reach areas that may have some decay, 1/4-in.holes can be drilled in the wood and filled, preferablyseveral times, with preservative solution that will diffuseinto the adjacent wood. The holes should then beplugged with preservative-treatad wood dowels.

NOTE: Most preservatives should not be usedindoors. Always check manufacturer’s recommen-dations and Material Safety Data Sheets for properuse of a l l preservat ives .

The use of berates to control wood decay is a relativelynew development in the United States and may beparticularly useful on log structures with moisture prob-lems. The material is manufactured for brush or sprayapplication or as a “rod” that can be inserted into holesbored in the wood. The material diffuses through wetwood and provides protection against decay and wood-destroying insects. This material will probably beavailable through distributors and can be applied byprofessionals such as pest control operators, log cabinmanufacturers, and pole treaters.

Structures in Marine Environments

The marine environment is particularly harsh on woodfinishes because of the abundance of moisture andexposure to full sunlight. The natural weatheringprocess of wood and finishes is accelerated.

For best protection, any wood fully exposed to marineenvironments, especially if it is in contact with the wateror soil, should be pressure treated with a wood preser-vative. The treatment should be based on applicablemarine or in-ground use specifications and local-useexperience. Such treated woods are not alwayspaintable. However, wood treated with waterborne pre-servatives is paintable when clean and dry. Chromatedcopper arsenate (CCA) is the most paintable of thewaterborne preservatives. Protected wood, in whichserious wood decay is not expected to occur, can betreated with a paintable water-repellent preservative,coated with a suitable paint primer, and topcoated withat least two coats of quality, exterior finish. Oils andother penetrating natural finishes will need to bereapplied at least annually and often semiannually, de-pending on exposure conditions. Antifouling paints canbe used for protection against marine organisms onpiers and boat hulls.

Natural film-forming finishes (varnishes) for woodsexposed to marine environments need almost constantcare and refinishing. If used, varnishes should be

Figure 30---Extensive damage to a wooden waterfrontstructure from marine borers (Limnoria).

applied in three- to six-coat thicknesses for bestperformance. The application of a paintable water-repellent preservative or pigmented, varnish-compat-ible stain before finishing will help improve the perfor-mance of the varnish.

Wood used in salt or brackish water can also bedamaged by marine borers (Fig. 30). Marine borersare serious pests in wooden waterfront structures andhave damaged wooden vessels throughout history.The marine borers that cause the greatest amount ofdamage can be divided into two main groups:Mollusca and Crustacea. Molluscs are shell animalslike clams and oysters. Molluscan borers are dividedinto two families, Teredinidae and Pholadidae.Teredinidae contains the genera Teredo and Bankia,commonly called wood-boring shipworms. Pholadidaeincludes the genus Martesia and resembles clams.This family is commonly referred to as Pholads or rock-boring piddock. Marine borers in the phylum Crustaceaare related to lobsters and crabs. Limnoria andSphaeroma are important wood-destroying genera inthis group of borers. Limnoria are commonly referredto as gribbles.

Marine borers vary greatly in their distribution andability to destroy wood. They are generally moredestructive in tropical waters. Their population can rise

42

and fall depending on any number of factors such assalinity changes resulting from floods or other causes,water temperatures, and dissolved oxygen content.There is some indication that marine borer populationsincrease as pollution levels decline. Because of thevariation in marine borers, consult local authoritiesbefore purchasing wood that will be placed in salt orbrackish water.

Treated Wood

Preservatives

Lumber, plywood, and other wood products pressuretreated with preservatives do not normally requirefinishing for outdoor exposure. However, if a finish isdesired for esthetic reasons, certain precautions shouldbe exercised because each of the three classes ofwood preservatives imparts certain characteristics tothe wood that affect its ability to accept and retain afinish. The general classes of wood preservatives are(1) preservative oils such as coal-tar creosote,(2) organic solvent solutions such as pentachlorophe-nol, and (3) waterborne preservatives.

When coal-tar creosote or pentachlorophenol in aheavy oil solvent with low volatility is used to pressuretreat wood, successful finishing is impossible. Thesurface is generally oily and dark, and the dark colorwill usually bleed through any paint. Poor adhesion isalso likely. If the wood has weathered for years, it cansometimes be stained or painted, but it is best to finisha small area first and expose it to direct sunlight todetermine if the finish would perform well.

When wood has been pressure treated with pentachlo-rophenol in a highly volatile solvent such as methylenechloride or liquefied petroleum gas, its surface can stillgenerally be painted but only after the solvent hascompletely evaporated. However, solvents vary in theirvolatility, and the evaporation period may take from 1 to2 years. Even with special drying schedules, thecomplete paintability of the wood may not be restored.Similarly, wood that is pressure treated with a water-repellent preservative is loaded with relatively largequantities of the preservative and solvent, and thesolvent must be completely removed before painting isattempted. This process, however, should not beconfused with the brush, spray, or dip treatment of awater-repellent preservative, which serves to enhancepaint durability and wood performance.

Coal-tar creosote and pentachlorophenol are restricted-use pesticides and therefore the solutions are notavailable to the general public. However, wood treatedwith these preservatives is available either as new orused material, such as railroad ties and poles. The

Consumer Information Sheets of the EnvironmentalProtection Agency specify that wood treated with eitherof these preservatives should not be used wherefrequent or prolonged contact with the skin will occur orin residential, industrial, or commercial interiors, exceptfor laminated beams or building components that are inground contact. When used in these applications, twocoats of an appropriate sealer are required. Becauseof these restrictions, coal-tar creosote and pentachloro-phenol are not now commonly used in or aroundresidential or commercial buildings.

Waterborne preservatives, such as chromated copperarsenate (CCA), are the only common preservativechemicals applied by a pressure process that do notadversely affect the wood finishing characteristics. Infact, those preservatives that contain chromium reducethe degrading effect of weathering. Lumber or plywoodtreated in this manner can be finished following normalprocedures or allowed to slowly weather to a light gray.Water-repellent treatments should be used for decksand fences. This treatment will help reduce surfacechecking and cracking.

Most pressure-treated construction lumber and ply-wood in the southeastern United States is producedfrom southern yellow pine with flat-grained characteris-tics and wide latewood bands. Because of the rela-tively poor paint-holding properties of this speciesgroup, semitransparent oil-based penetrating stains orother penetrating finishes will give better service thanwill other finishes. The finish should be applied afterthe wood has thoroughly dried. Drying may take daysor weeks depending on how wet the wood is initiallyand on drying conditions.

Wood that is pressure treated with waterborne preser-vatives generally contains large quantities of waterwhen shipped to retail lumber yards. Therefore, careshould be exercised to make certain that the lumber isdry before finishing. Air drying in place is acceptable,although some shrinking, warping, and checking mayresult. Regardless of the finish to be applied, therecommended procedures for its application should befollowed carefully, and the wood should be completelyclean.

NOTE: The Environmental Protection Agency (EPA)has issued consumer information sheets on each ofthe three general classes of wood preservatives;creosote, pentachlorophenol, and the inorganicarsenical, which include chromated copper arsen-ate (CCA), ammoniacal copper arsenate (ACA), andammoniacal copper zinc arsenate (ACZA). Theconsumer information sheet on arsenical is in-cluded as Appendix B.

43

For maximum service life, wood items used outdoors,such as lawn furniture, playground equipment, planters,and retaining walls, should be pressure treated with apreservative before finishing, especially if the item is insoil contact. Naturally durable woods may be suitablefor mild exposures. Water-repellent preservatives andsemitransparent penetrating stains will generallyprovide the longest service life coupled with ease ofrefinishing.

Caution: For items where human contact is likelyor where food will be served, make certain that thewood preservative and finishing system is accept-able for that application.

Fire-Retardant Treatments

Treating wood with fire retardants should not interferewith adhesion of decorative paint coatings unless thehygroscopicity of the wood has been increased be-cause of the nature of the chemical used. Only thosefire-retardant treatments specifically prepared andrecommended for outdoor exposure should be usedoutdoors; others may leach from the wood, causingdiscoloration and early paint failure. Treated woodsare generally painted according to the manufacturer’srecommendations rather than left unfinished becausethe chemical treatment and subsequent kiln-dryingprocess often darken and irregularly stain the wood.

Southern yellow pine and Douglas-fir are two speciescommonly treated with fire-retardant chemicals.Because these species normally do not hold paint orsolid-color stains well, strict adherence to recom-mended finishing procedures should be followed.Treated western hemlock and ponderosa pine havesomewhat better finishing characteristics than those ofsouthern yellow pine and Douglas-fir.

Because of the variability of wood preservatives andfire retardants as well as the nature of the treatingprocesses, the manufacturer should be consultedregarding finishing details. The manufacturer usuallyhas specific recommendations for achieving maximumservice life for paint and other finishes. Wood treatedin strict adherence to recognized standards shouldcontain a quality stamp indicating the treatment,treating company, and inspection bureau.

Refinishing of Wood

Exterior wood surfaces should be refinished only as theold finish deteriorates or for esthetic reasons such as achange in color or type of finish. Too frequent refinish-ing, especially with paint, leads to a finish buildup andsubsequent cracking and peeling. In some cases, dirty

painted surfaces can be cleaned by washing with amild detergent and water. To achieve maximumservice life from a refinished surface, special surfacepreparation and finish application techniques should befollowed.

Opaque Finishes

In refinishing an old paint coat or a solid-color stain thathas weathered normally, proper surface preparationand cleaning are essential for optimal performance ofthe new finish. If the surface had been finished with alead-based paint, be sure to follow necessary precau-tions for preparing the surface (see section on lead-based paint).

Latex paint can be applied to weathered paintedsurfaces if the old paint is clear and sound. Thepaintability of the surface can be tested with a simpleprocedure. After cleaning the surface, refinish a small,inconspicuous area with latex paint, and allow it to dryat least overnight. To test for adhesion, firmly pressone end of an adhesive bandage onto the paintedsurface. Then, jerk the bandage off with a snappingaction. If the tape is free of paint, the paint is wellbonded and the old surface does not need to be primedor cleaned (Fig. 31). If the paint adheres to the tape,the old surface is too chalky and needs priming with anoil-based primer or additional cleaning. The primershould penetrate the old chalky surface and form a firmbase for the new coat of paint. If both the freshlyapplied paint and the old paint coat adhere to the tape,the old paint is not well bonded to the wood and mustbe removed before repainting the surface.

To refinish the old surface, first scrape away all loosecoating. Sand any remaining coating to “feather” theedges smooth with the bare wood. Then, scrubremaining coating with a brush or sponge and water.Rinse the scrubbed surface with clean water. Wipe thesurface with your hand. If the surface is still dirty orchalky, scrub it again with a detergent. Mildew shouldbe removed with a dilute solution of household bleach.Rinse the cleaned surface thoroughly with fresh waterand allow it to dry before recoating. Areas of exposedwood should be treated with a water-repellent preser-vative and allowed to dry for at least two sunny daysand then primed. Wipe away any water-repellentpreservative accidentally applied to coated areas.Apply topcoat or topcoats. When refinishing with oil-based coatings, one topcoat is usually adequate if theold surface is still in good condition.

It is particularly important to clean areas protected fromsun and rain such as porches and sidewalls with wideroof overhangs. These areas tend to collect dirt andwater-soluble materials that interfere with adhesion of

Figure 31—The adhesive bandage test can be used todetermine if a new coat of paint is properly bonded toan old surface. The bandage on the right was appliedto a well-bondedpaint coat, the bandage on the left toa poorly bonded paint coat.

the new coating. Protected areas can be refinishedevery other time exposed areas of the house arerefinished.

Natural Finishes

Water-Repellent Preservatives

Water-repellent preservatives can be renewed by asimple cleaning of the old surface with a bristle brushand application of a new coat of finish. In some cases,a mild scrubbing with a detergent followed by rinsingwith water is appropriate. The second coat of water-repellent preservative will last longer than the first;more preservative can be applied because of smallsurface checks that are created by weathering.

Before renewing a water-repellent preservative, makecertain that there is no mildew on the old surface. Ifmildew is present, it can grow through the new coating.See section on prevention and removal of mildew.

To determine if a water-repellent preservative has lostits effectiveness, splash a small quantity of wateragainst the wood surface. If the water beads up andruns off the surface and does not penetrate it, thetreatment is still effective. If the water soaks in, thewood may or may not need to be refinished. Some-times a water-repellent preservative breaks down at thesurface but is still effective. In this case, the surface ofthe wood appears wet (dark) but the moisture does notpenetrate deeply. Refinishing is also required when

the wood surface starts to show blotchy discolorationcaused by extractives or mildew.

Oils

Oil finishes can be renewed following the suggestionsgiven for water-repellent preservatives.

Semitransparent Penetrating Stains

Semitransparent penetrating stains are relatively easyto refinish. Excessive scraping and sanding are notrequired—simply use a stiff bristle brush to remove allsurface dirt, dust, and loose wood fibers, and thenapply a new coat of stain. The second coat of pen-etrating stain often lasts longer than the first becausemore can be applied.

Note: Steel wool and wire brushes should neverbe used to clean surfaces to be finished withsemitransparent stains, oils, or water-repellentpreservatives because small iron deposits may beleft on the surfaces. These deposits can react withcertain water-soluble extractives in wood likewestern redcedar, redwood, Douglas-fir, and theoaks to produce dark blue-black stains on thesurface. in addition, additives present in somesemitransparent penetrating stains and water-repellent preservatives may cause surface iron tocorrode. The corrosion products may then reactwith certain wood extractives to form a blue-black,unsightly discoloration that becomes sealedbeneath the new finishing system.

Transparent Film-Forming Coatings

The refinishing practices described for opaque coatingsshould generally be followed for transparent, film-forming finishes such as varnish. For oils and similarpenetrating finishes, follow the practices recommendedfor water-repellent preservatives.

Removal of Finish

The removal of paint and other film-forming finishes isa time-consuming and often difficult process. However,it is sometimes necessary for the preparation of a newsurface if, for example, the old surface is covered withseverely peeled or blistered paint, or if cross-graincracking has occurred from excessive paint buildup.The removal of finish is also necessary if a penetratingstain or water-repellent finish is to be applied to apreviously painted or stained (solid-color) surface.Finishes can be removed by sanding, sandblasting,spraying with pressurized water, using electricallyheated paint removers and blow torches, or strippingwith chemicais. Consult with local equipment-rental

stores and paint dealers for available equipment, orrequest bids from professional contractors. Beforeproceeding, be sure to review the section on theidentification of health hazards associated with theremoval of lead-based paints.

Sanding

Disk or siding sanders equipped with a tungstencarbide abrasive disk of medium grit are effective inremoving old paint. This method is faster than others,and the tungsten carbide disk is less likely to clogcompared to conventional sanding disks. The depth ofcut for the sander can be set with the siding guide, butexperienced operators often work freehand, without theguide. The operator should be careful to remove onlythe paint and not excess wood. After finishing with thedisk sander, it is desirable to smooth the surfacesomewhat by light manual sanding or with a straight-Iine power sander using 120 grit in the direction of thewood grain.

Wet Sandblasting and Pressurized Water Spray

Paint can also be removed by blasting with wet sand orusing a high-pressure water spray. These methodsusually require the services of a professional. Thesand particles or water can erode the wood as well asstrip the paint. The softer earlywood is eroded fasterthan the latewood, resulting in an uneven, roughsurface. These rough surfaces may not be suitable forpainting. The pressurized water spray method usesapproximately 600 to 2,000 pounds of pressure persquare inch.

Caution: Some old paints contain lead, and sand-ing, sandblasting, or disturbing the surface in anyway will release tiny lead particles into the air.Inhalation of these lead particles is detrimental tohealth. Be sure to read the section on lead-basedpaints. For the sake of safety, anyone sandblastingor using pressurized water spray equipment shouldwear approved eye goggles as well as a dust maskor respirator, as appropriate. Electrical equipmentshould be double insulated or equipped with athree-wire grounded outlet.

Heat

Electrically heated paint removers can be used to strippaint. The heater simply heats and softens the paint,causing it to separate from the wood. This method,although effective, is slower than sanding and requiresat least a 1,000-W heater to be effective. Paint canalso be removed with an open-flame blowtorch. Ablowtorch is effective and inexpensive. However,there is a constant danger of starting a fire within

the walls of the building from flames that penetratecracks in the siding. This method is usually bestdone by professionals.

Chemicals

Liquid paint and varnish removers, such as commer-cially prepared chemical mixtures, lye, or trisodiumphosphate, will also remove paint from wood surfaces.After the paint is removed, the surface sometimes mustbe neutralized; before repainting, the wood surfaceshould be sanded in the direction of the grain. Strongcaustic solutions, such as lye and trisodium phosphate,may leave the wood surface very porous.

Failure or Discolorationof Finish

Paint properly applied and exposed under normalconditions is usually not affected by the first 2 to 3 yearsof exposure. Areas that deteriorate the fastest are thoseexposed to the greatest amount of sun and rain, usuallyon the south and west sides of a building.

Under normal conditions, paint deteriorates first bysoiling or accumulating slight traces of dirt. Next, thecoating gradually starts to chalk and erode away. Paintcan sometimes become discolored by mildew, bluestain, wood extractives, pitch, and metals, makingrepainting necessary. In these cases, however, asimple repainting will not correct the problem for long.Furthermore, frequent repainting is expensive, and abuildup of paint on the wood surface may lead to cross-grain cracking or other severe paint failures. If the oldpaint surface is not properly cleaned before repainting,intercoat peeling may also result. Paint may also peelfrom poor construction practices that allow excessmoisture buildup in the wood. Iron and water stain maybe a problem when natural finishes are applied andpoor construction practices have been followed.

Finish failure or discoloration can be eliminated only byidentifying and correcting the problem. Refinishingwithout correcting the original problem will result inrepeated failure of the finish.

Mildew

Mildew is probably the most common cause of discol-oration of house paint, solid-color and semitransparentstains, and natural finishes (Fig. 32). it also causes thegray discoloration of unfinished wood. The termmildew applies both to the fungus (a type of micro-scopic plant life) and to its staining effects on thesubstrate (the substance on which it grows; in thiscase, the coating and the wood). Mildew grows on the

46

Figure 32—Mildew on paint is most common inwarm, humid climates. It also occurs in shadedor protected areas.

paint or wood surface and does not normally degradethe wood as do wood-rotting fungi. The most commonmildew species are black, but some are brown, red,green, or other colors, Mildew grows most extensivelyin warm, humid climates. Although mildew may befound anywhere on a building, whether or not thebuilding is painted, it is most commonly found on wallsbehind trees or shrubs where air movement is re-stricted. Mildew may also be associated with the dewpattern of the house. Dew will form on those parts ofthe house that are not heated and tend to cool rapidly,such as eaves, the ceilings of carports and porches,and the wall area between studs. The dew provides asource of moisture for the mildew.

Mildew can be distinguished from dirt by examinationunder a high-power magnifying glass. In the growingstage, when the paint surface is damp or wet, a mildewfungus is characterized by its threadlike growth. In thedormant stage, when the surface is dry, the fungus hasmany egg-shaped spores. By contrast, granular par-ticles of dirt are irregular in size and shape. A simpletest for the presence of mildew on paint and wood canbe made by applying a drop or two of a fresh solutionof household liquid bleach (5 percent sodium hypochlo-rite) to the stained area. The dark color of mildew willusually bleach out in 1 or 2 minutes. Discoloration thatdoes not bleach out is probably dirt. Fresh bleach solu-tion should be used because the solution deterioratesupon standing and loses its potency.

Effect of Paint Makeup

Some paints are more vulnerable than others to attackby mildew fungi. Zinc oxide, a common paint pigment

in topcoats, inhibits the growth of mildew (that is, actsas a mildewstat), whereas titanium dioxide, anothercommon paint pigment, has very little inhibitory effect.

With oil-based paints, mildew progresses more readilyon exterior flat house paint than on exterior semiglossor gloss enamel; paints or stains containing linseed oilare very susceptible to mildew. Of the available water-based paints, acrylic latex is the most resistant tomildew. Porous latex (water-based) paints without amildewcide, applied over a primer coat with linseed oil,will develop severe mildew in warm, humid climatescommon in the South.

Mildewcides, poisons for mildew fungi, are oftenadded to paints. The paint label should indicate if amildewcide is present in the paint. If it is not, amildewcide can sometimes be added by the localpaint dealer. When properly applied to a cleansurface, paint containing a mildewcide should preventmildew for several years.

Prevention and Removal

For new wood surfaces in warm, humid climates,mildew can be prevented by using topcoats of paintcontaining zinc oxide and mildewcide over a primercoat that also contains a mildewcide. For mild cases ofmildew, use a paint containing a mildewcide.

Mildew must be killed before wood is refinished, or themildew will grow through the new paint coat or solid-color stain. To kill mildew, scrub the painted surfacewith a bristle brush or sponge and the following solution:

1/3 c household detergent

1 to 2 qt (5 percent) sodium hypochlorite(household bleach)

2 to 3 qt warm water

This mixture can also be used to remove mildew fromnaturally finished or unfinished wood.

WARNING: Do not mix liquid household bleachwith ammonia or with any detergents or cleanserscontaining ammonial Bleach and ammonia incombination are lethal, similar to mustard gas.People have died from breathing the fumes fromsuch a mixture. Many household cleaners containammonia, so be extremely careful what type ofcleaner is mixed with bleach.

If mildew has formed on an earlier coat of finish andsubsequent coats have been applied without firstcleaning the surface, the mildew can grow through thenew coating. In this situation it is usually impossible tocontrol the mildew. The entire paint coat should bestripped and a new finishing system applied.

47

Peeling and Cracking

Intercoat Peeling

Intercoat peeling is the separation of the new paint coatfrom the old, which indicates that the bond between thetwo coats is weak (Fig. 33). Intercoat peeling usuallyresults from inadequate cleaning of the weatheredsurface prior to repainting. It generally occurs within1 year of repainting. This type of paint peeling can beprevented by good cleaning and painting practices.Intercoat peeling can also occur on freshly paintedwood if too much time elapses between application ofthe primer coat and topcoat. If more than 2 weekselapse before a topcoat is applied to an oil-basedprimer, soap-like materials may form on the surfaceand interfere with the bonding of the topcoat. Whenthe period between applications exceeds 2 weeks, thesurface should be scrubbed before applying the secondcoat. If the primer coat is applied in the fall, do notwait until spring to apply the topcoat. If the lapse intime is unavoidable, the old primer paint should bewashed and the surface reprimed before painting.

Cross-Grain Cracking

Cross-grain cracking occurs when paint coatingsbecome too thick (Fig. 34). This problem often occurson older homes that have been painted several times.Normally, paint cracks in the direction it was brushedonto the wood (that is, with the grain). Cross-graincracks run across the grain of the wood and paint.Once cracking has occurred, the only solution is tocompletely remove the old paint and apply a newfinishing system on the bare wood.

To prevent cross-grain cracking, follow the paintmanufacturer’s recommendations for spreading rates.Do not repaint unweathered, protected areas such asporch ceilings and roof overhangs as often as the rest

Figure 33---Intercoat peeling of paint is usually causedby poor preparation of the old surface.

of the house. If possible, repaint these areas only asthey weather and require new paint. If repainting isrequired, be sure to scrub the areas with a sponge orbristle brush and detergent in water to remove anywater-soluble materials that will interfere with adhesionof the new paint. Latex paints, based on either vinyl oracrylic polymers, have not been known to fail by cross-grain cracking.

Blistering

Temperature blisters are bubble-like swellings thatoccur on the surface of the paint film as early as a fewhours or as long as 1 to 2 days after painting. Theyoccur only in the last coat of paint (Fig. 35). Theblisters are formed when a thin, dry skin forms on theouter surface of the fresh paint and the liquid thinner inthe wet paint under the skin changes to vapor andcannot escape. When the direct rays of the sun fall onfreshly painted wood, the rapid rise in temperaturecauses the vapors to expand and produce blisters.Usually, only oil-based paint blisters are formed in thisway. Dark colors that absorb heat and thick paint coatsare more likely to blister than are white paints or thincoats.

To prevent temperature blisters, avoid painting sur-faces that will soon be heated. The best procedure isto Yellow the sun around the house.” The north side ofthe building should be painted early in the morning, theeast side late in the morning, the south side well intothe afternoon, and the west side late in the afternoon.However, at least 2 hours should be allowed for thefresh paint film to dry before it cools to the point wherecondensation could occur. If blistering does occur,allow the paint to dry for a few days, scrape off theblisters, smooth the edges with sandpaper, and spotpaint the area.

Figure 34---Cross-grain cracking results from anexcessive buildup of paint.

Figure 35---Temperature blisters can form whenpartially dried paint is suddenly heated by thedirect rays of the sun. (M 147 706-5)

Moisture Blisters

Moisture blisters are also bubble-like swellings on thesurface of the paint film. As the name implies, theyusually contain moisture when they are formed. Mois-ture blisters may occur where outside moisture such asrain enters the wood through joints and other end-grainareas of boards and siding. Moisture may also enterbecause of poor construction and maintenance prac-tices, particularly in the lower courses of siding. Paintfailure is most severe on the sides of buildings that facethe prevailing winds and rain. Damage appears afterspring rains and throughout the summer. Moistureblisters may occur in both heated and unheatedbuildings.

Moisture blisters may also result from the movement ofwater vapor from within the house to the outside.Plumbing leaks and improper venting of bath andkitchen areas, clothes dryers, and humidifiers aresources of inside water. If the warm side of outsidewalls does not contain a vapor barrier, the moisture willmove through the wall and result in moisture blisters orpaint peeling. Such damage is not seasonal andoccurs when the faulty condition develops.

Moisture blisters usually include all paint coats down tothe wood surface. After the blisters appear, they dryout and collapse. Small blisters may disappear com-pletely, but fairly large ones may leave a rough spot; insevere cases, the paint will peel (Fig. 36). Thin coat-ings of new, oil-based paint are the most likely toblister. Old, thick coats are usually too rigid to swelland form blisters, so cracking and peeling will resultinstead.

Elimination of moisture and use of a vapor barrier arethe only practical ways to prevent moisture blisters inpaint. The moisture source should be identified and

eliminated to avoid more serious problems such aswood decay (rot) and loss of insulating value.

Discoloration

In some species, such as western redcedar, cypress,and redwood, the heartwood is dark because of thepresence of water-soluble extractives. The extractivesgive these species their attractive color, good stability,and natural decay resistance, but they can also dis-color paint. The heartwood of Douglas-fir and southernyellow pine can also produce staining extractives,although the problem is not as severe as that encoun-tered with western redcedar, cypress, and redwood.

When extractives discolor paint, moisture is usually theculprit. The extractives are dissolved and leached fromthe wood by water. The water then moves to the paintsurface, evaporates, and leaves the extractives behindas a reddish-brown stain. The latex paints and the so-called breather or low-luster paints are more porousthan conventional paints and thus more susceptible toextractive staining.

Figure 36---Paint can peel from wood when excessivemoisture moves through the house wall. Some cross-grain cracking is also evident on this older house. (Photocourtesy of Southern Forest Products Association.)

49

Figure 37---Streaked discoloration from water-soluble extractives. Discoloration can resultfrom water wetting the back of one piece ofsiding and then running down the front of thenext piece (top). Water that causes discolora-tion can also lead to paint failure (bottom).(MC83 9028, MC83 9027)

Diffused Discoloration

Diffused discoloration from wood extractives is causedby rain and dew that penetrate a porous or thin paintcoat. It may also be caused by rain and dew thatpenetrate joints in the siding or by water from faultyroof drainage and gutters, Diffused discoloration isbest prevented by following good painting practices.

leaks, faulty gutters, or rain blown through louvers invents.

Streaked discoloration can be prevented by reducingcondensation or the accumulation of moisture in thewalls. A vapor barrier (such as a continuous 6-milpolyethylene sheet) should be installed on the inside ofall exterior walls in new or remodeled houses located inclimate zones where winter condensation can occur inwalls. If a vapor barrier is not practical, the inside of allexterior walls should be finished with a vapor-resistantpaint. Water vapor in the house can be reduced byventing exhaust fans in bathrooms and kitchens to theoutside. Clothes dryers should also be vented to theoutside and not to the crawlspace or attic. Humidifiersshould be avoided, and if the house contains acrawlspace, the soil should be covered with a vaporbarrier to prevent migration of water into the livingquarters (see section on control of moisture content).

Rain water can be kept from entering the walls byproper maintenance of gutters and roof. For colderareas in the South, condensation in the attic can beprevented by installing adequate insulation and provid-ing proper ventilation. For gable roofs, vents should beprovided at the gable ends and should be about 1/300of the ceiling area. More positive air movement can beobtained if additional openings are provided in theoverhang. Hip roofs should have air-inlet openings inthe louvers and several smaller roof vents near theedges.

To stop extractive discoloration, moisture problemsmust be eliminated. Streaked discoloration will usuallyweather away in a few months. However, discolorationin protected areas can become darker and moredifficult to remove with time. In these cases, discoloredareas should be washed with a mild detergent soonafter the problem develops. Paint cleaners are alsoeffective on darker stains. A solution containing 4 to16 oz of oxalic acid per gallon of warm water may alsobe effective.

Streaked DiscolorationWax Bleed

streaked discoloration or rundown can also occur whenwater-soluble extractives are present in the wood (Fig. 37).This discoloration results when the back of the siding iswetted, the extracfives are dissolved, and the colored waterruns down the face of the adjacent painted boards fromthe lap joint. Streaked discoloration can also resultfrom water vapor within the house moving to theexterior walls and condensing during cold weather.Major sources of water vapor are humidifiers, clothesdryers and showers that are not vented to the outside,normal respiration, and moisture from cooking anddishwashing. Streaked discoloration may also becaused by water draining into exterior walls from roof

The petrolatum wax used as a water repellent in somehardboard siding products may cause a discolorationproblem called wax bleed. This problem can often becorrected by washing the affected surface with adetergent and water. For more severe cases, thesurface should be reprimed and topcoated afterthorough washing and rinsing.

Chalking

Chalking results when a paint film gradually weathers ordeteriorates, releasing individual particles of resin and

50

pigment. These particles act like a fine powder on the paintsurface. Most paints chalk to some extent, which is desirablebecause chalking cleans the painted surface. However,chalking is objectionable when it washes down over asurface with a different color (Fig. 38) or when it causespremature disappearance of the paint film through excessiveerosion. With colored or tinted paints, chalking is a commoncause of fading.

Discoloration problems from chalking can be reducedby selecting paint with slow chalking tendencies. Themanner in which a paint is formulated may determinehow fast it chalks. Therefore, if chalking is likely to bea problem, select a paint that the manufacturer hasindicated will chalk slowly.

For repainting surfaces that have chalked excessively,proper preparation of the old surface is essential if thenew paint coat is expected to last. Scrub the oldsurface thoroughly with a detergent solution to removeall old deposits and dirt. Rinse thoroughly with cleanwater before repainting. The use of a top-quality oil-based primer or a stain-blocking acrylic latex primermay be necessary before latex topcoats are applied.Otherwise, the new paint coat will peel. Discolorationor chalk that has run down on a lower surface mayberemoved by vigorous scrubbing with a good detergent.This discoloration will usually gradually weather away ifthe chalking problem on the painted surface has beencorrected.

Exudation of Pitch

Pine and Douglas-fir can exude pitch (resin), and cedarspecies (except western redcedar) can exude oils.Pitch and oils are not ordinarily a problem becauselumber manufacturers have learned how to “set” pitchand evaporate excess oil during the kiln-drying pro-cess. The material is simply planed or sanded later inthe manufacturing process and does not presentadditional problems. However, where the properschedules are not used in drying the lumber, problemscan result,

When exudation occurs before the wood has beenpainted, the exuded materials should be removed. Ifthe exuded pitch has hardened, it can be removedfairly easily with a putty knife, paint scraper, or sandpa-per; however, if it is still soft, such procedures smearthe pitch over the surface of the wood. If the pitch isnot removed, the paint is likely to alligator, crack, andfail over the pitch-coated areas soon after painting.Soft pitch should be removed thoroughly by scrubbingthe surface with cloths wet with denatured alcohol.After most of the pitch has been scrubbed off, thesurface should be sanded. Any further exudation thatoccurs before subsequent coats of paint are applied

Figure 38—Chalking of paint. Some paints or stainschalk badly. Chalking can discolor a lower surfaceas the resin and pigment particles wash down.

should be removed by scrubbing the surface withalcohol.

If exudation occurs after the wood is painted, the woodmight best be left alone until it is time to repaint. Thewood should then be scraped thoroughly before newpaint is applied. If a few boards in the structure areparticularly unsightly because of exudation or becauseof early paint failure, it may be wise to replace themwith new lumber before repainting.

Exudation of pitch is favored by fluctuations in tempera-ture or by high temperatures. In extreme cases,boards have been known to continue to exude pitch formany years (Fig. 39). Repainting should be deferreduntil all exudation has ceased or until repainting hasbecome necessary for other reasons. No paints andpainting procedures can be relied upon to preventexudation of pitch.

Stains

Stains can result from mechanical and natural causes.In this section, we will discuss iron stains, fungal bluestain, brown stain over knots, and water stain.

Iron Stains

Two types of stains are associated with iron. The red-brown discoloration caused by rust is associated withthe use of ferrous nails and metal screens. Anotherstain, a blue-black discoloration, is caused by the

51

Figure 39---Exuded pitch from knots and pitch pocketon siding. (M84 0493-11)

Figure 40---Metal fasteners or window screens cancorrode and later discolor paint as leaching occurs.(M84 0490-4)

reaction of wood extractives with iron and is associatedwith the use of ferrous nails and with iron traces fromtools.

Rust. When standard ferrous nails are used onexterior siding and the siding is painted, a red-brown

discoloration may appear through the paint in theimmediate vicinity of the nailhead. To prevent ruststains, use corrosion-resistant nails. These includehigh-quality galvanized, stainless steel, and aluminumnails. The heads of poor-quality galvanized nails canbe chipped when they are driven into the siding,corrode easily, and, like ferrous nails, cause unsightlystaining of the paint. If rust is a serious problem on apainted surface, the nails should be countersunk andcaulked, and the area should be spot primed and thentopcoated.

Rust stains may also occur when standard ferrous nailsare used in association with other finishing systemssuch as solid-color or opaque stains, semitransparentpenetrating stains, and water-repellent preservatives.Rust stains can also result when screens and othermetal objects that are subject to corrosion and leachingare fastened to the surface of the building (Fig. 40).

Chemical stains. An unsightly blue-black discolora-tion of wood can be caused by the chemical reaction ofwood with iron. in this case, the iron reacts with certainwood can be caused by the chemical reaction of woodwith iron. In this case, the iron reacts with certain woodextractives, such as tannins or tannic acid in cedar,redwood, or oak. Ferrous nails and other iron append-ages are the most common source of iron in chemicalstaining (Fig. 41), but problems have also been associ-ated with traces of iron left from cleaning the woodsurface with steel wool, wire brushes, or even irontools. The discoloration can sometimes becomesealed beneath a new finishing system.

A solution of oxalic acid in water will remove blue-blackdiscoloration providing it is not already sealed beneatha finishing system. The stained surface should begiven several applications of a solution containing1/2 to 1 lb of oxalic acid per gallon of hot water. Afterthe stains disappear, the surface should be thoroughlywashed with fresh warm water to remove the oxalicacid and any traces of the chemical causing the stain.if all sources of iron are not removed or protected fromcorrosion, staining re-occurs.

WARNING: Oxalic acid and its water solution aretoxic and should be used with great caution.

Blue Stain

Blue stain is caused by microscopic fungi that com-monly infect the sapwood of all woody species. Al-though microscopic, they produce a blue-black discol-oration of the wood. Blue stain does not normallyweaken wood structurally, but conditions that favorstain development are also ideal for serious wooddecay and paint failure.

52

most cases, the resin is “set” or hardened by the hightemperatures used in kiln drying of construction lumber.

Figure 41—Blue-black discoloration resulting fromthe use of ferrous nails. (M84 0491)

Wood in service may contain blue stain, and no detri-mental effects will result as long as the moisturecontent is kept below 20 percent. Wood in properlydesigned and well-maintained houses has a moisturecontent of less than 20 percent. However, if the woodis exposed to moisture from rain, condensation, orleaky plumbing, the moisture content will increase, theblue- stain fungi will develop further, and decay mayeven follow,

To prevent blue stain from discoloring paint, followgood construction and painting practices. First, dowhatever is possible to keep the wood dry. Provide anadequate roof overhang, and properly maintainshingles, gutters, and downspouts. Window and doorcasings should slope out from the house, allowingwater to drain away rapidly. In northern climates, use avapor barrier on the interior side of all exterior walls toprevent condensation. Vent clothes dryers, showers,and cooking areas to the outside and avoid the use ofhumidifiers. Treat wood with a water-repellent preser-vative, then apply a nonporous mildew-resistant primer,and finally apply at least one topcoat of finish contain-ing a mildewcide. If the wood has already beenpainted, remove the old paint and allow the wood to drythoroughly. Apply a water-repellent preservative, andthen repaint the wood.

A 5-percent solution of sodium hypochlorite (ordinaryliquid household bleach) may sometimes remove blue-stain discoloration, but it is not a permanent cure. Besure to use a fresh solution of bleach as its effective-ness can diminish with age. The moisture problemmust be corrected to obtain a permanent cure fordiscoloration.

Brown Stain Over Knots

The knots in many softwood species, particularly pine,contain an abundance of resin. The resin can some-times cause paint to peel or turn brown (Fig. 42). In

Good painting practices should eliminate or controlbrown stain over knots. First apply a good primer tothe bare wood and then apply two topcoats. Forexterior wood, do not apply ordinary shellac or varnishto the knot area initially because this may result in earlypaint failure. Specialty primer paints are available toseal knots for outdoor painting. These primer paintswork best under high-quality acrylic latex topcoatpaints.

Water Stain

Wood siding can become water stained, particularly if itis left unfinished or if a natural finish has started todeteriorate (Fig. 5). Water stain is most common at thebase of sidewalls where rainwater runs off a roof, hits ahard surface, and splashes back onto the side of thebuilding. The water causes the finish to deterioratequickly in this area. If the finish is not replaced, thewater can begin to remove the water-soluble extrac-tives, which accelerates the weathering process, andthe area becomes stained. Water stain can also beseen where gutters overflow. Good constructionpractices that keep water from contacting the woodshould be followed whenever possible to prevent waterstain. The wood should be treated regularly with awater-repellent preservative. Removing water stainscan be very difficult. Sometimes scrubbing with milddetergent and water is effective. Light sanding may beeffective on smooth wood surfaces. Bleaches such asliquid household bleach or oxalic acid solutions havebeen used with varying degrees of success.

Frgure 42—Brown discoloration of paint caused byresin exudation from a knot. (M84 0492-4)

53

Sources of Information onExterior Wood Finishings

54

The pesticides—wood preservatives,mildewcides, and fungicides---described inthis report were registered for the usesdescribed at the time the report was pre-pared. Registrations of pesticides are underconstant review by the EnvironmentalProtection Agency. Therefore, consult aresponsible State agency on the currentstatus of any pesticide. Use only pesticidesthat bear a Federal registration number andcarry directions for home and garden use.

Pesticides used improperly can be injuriousto humans, animals, and plants. Follow thedirections and heed all precautions on thelabel. Avoid inhalation of vapors andsprays; wear protective clothing and equip-ment if these precautions are specified onthe label.

If your hands become contaminated with apesticide, do not eat, drink, or smoke untilyou have washed. If you swallow a pesti-cide or if it gets in your eyes, follow the firstaid treatment given on the label and getprompt medical attention. If a pesticide getsonto your skin or clothing, remove theclothing immediately and wash skin thor-oughly.

Store pesticides and finishes containingpesticides in their original containers out ofthe reach of children and pets, under lockand key. Follow recommended practices forthe disposal of surplus finishing materialsand containers. Scraps of chemicallytreated wood or finished wood should neverbe burned, either for heat or for disposal.Toxic fumes may be released. Dispose ofthe material through ordinary trash collectionor burial.

Appendix A

Conversion Table for SI Units

55

Appendix B Handling Precautions

Consumer lnformation6

Inorganic ArsenicalPressure-Treated Wood(including: CCA, ACA, andACZA)

Consumer Information

This wood has been preserved by pressure-treatmentwith an EPA-registered pesticide containing inorganicarsenic to protect it from insect attack and decay.Wood treated with inorganic arsenic should be usedonly where such protection is important.

Inorganic arsenic penetrates deeply into and remains inthe pressure-treated wood for a long time. Exposure toinorganic arsenic may present certain hazards. There-fore, the following precautions should be taken bothwhen handling the treated wood and in determiningwhere to use or dispose of the treated wood.

Use Site Precautions

Wood pressure-treated with waterborne arsenicalpreservatives may be used inside residences as longas all sawdust and construction debris are cleaned upand disposed of after construction.

Do not use treated wood under circumstances wherethe preservative may become a component of food oranimal feed. Examples of such sites would be struc-tures or containers for storing silage or food.

Do not use treated wood for cutting-boards orcountertops.

Only treated wood that is visibly clean and free ofsurface residue should be used for patios, decks, andwalkways.

Do not use treated wood for construction of thoseportions of beehives which may come into contact withthe honey.

Treated wood should not be used where it may comeinto direct or indirect contact with public drinking water,except for uses involving incidental contact such asdocks and bridges.

Dispose of treated wood by ordinary trash collection orburial. Treated wood should not be burned in openfires or in stoves, fireplaces, or residential boilersbecause toxic chemicals may be produced as part ofthe smoke and ashes. Treated wood from commercialor industrial use (e.g., construction sites) may beburned only in commercial or industrial incinerators orboilers in accordance with state and Federal regula-tions.

Avoid frequent or prolonged inhalation of sawdust fromtreated wood. When sawing and machining treatedwood, wear a dust mask. Whenever possible, theseoperations should be performed outdoors to avoidindoor accumulations of airborne sawdust from treatedwood.

When power-sawing and machining, wear goggles toprotect eyes from flying particles.

After working with treated wood, and before eating,drinking, and using tobacco products, wash exposedareas of skin thoroughly.

If preservatives or sawdust accumulate on clothes,launder before reuse. Wash work clothes separatelyfrom other household clothing.

6 consumer information sheet approved by the U.S. Environmental

Protection Agency, January 1986.

56