Finishing Exterior PIywood, Hardboard andParticleboard

Daniel L. Cassens, Department of Forestry and Natural Resources, Purdue University, andWilliam C. Feist, Forest Products Laboratory, Forest Service, U.S. Department of Agriculture

Plywood, hard board and particleboard (wafer-board and chipboard) are manufactured or recon-stituted wood products as compared to solid wood(Figure 1). These products were introduced to theconstruction industry in the 1940s and are socommon today that nearly every new house has atleast some plywood, hardboard and particleboard init. Like solid lumber, these manufactured productswill give satisfactory performance if properlyfinished, but certain precautions must be taken —even before the building is constructed.

CARE AND PREPARATION BEFORECONSTRUCTION

Long life for an exterior wood finish begins duringconstruction. Building products, whether they beplywood, particleboard (waferboard and chip-board), hard board or solid lumber, should beprotected on the building site. If possible, store themin a cool, dry place and out of the sunlight andweather. If stored outside, keep the material off theground by stacking it on 4 × 4’s or similar material,and cover with polyethylene to prevent wetting byrain. Provide for good air circulation to preventcondensation under the plastic covering.

Plywood, hardboard and particleboard all tend toabsorb moisture through the ends and edges of thepanel faster than through the surface. While thepanels are still in a stack, seal the ends and edgeswith a liberal amount of a water-repellent preserv-ative or water repellent applied with a brush. Allow atleast two days for the water-repellent preservative orwater repellent to dry before painting. If the panel

faces are to be primed and painted, apply the samefinish to the edges. Any edges which are cut duringconstruction should also be sealed.

Ferrous nails can leave unsightly rust stains onplywood and other board products. Use high qualitygalvanized nails on surfaces to be painted. Alu-minum, stainless steel or high quality galvanizednails should be used if a water-repellent preserv-ative, water repellent or semitransparent penetrat-ing stain is to be used.

Regardless of the type of finish used, the surfaceto which it is applied should be clean and dry. Dofinishing work only in clear weather. Avoid applyingfinishes to exceedingly hot or cold surfaces. Paintshould not be applied to a cold surface which willsoon be heated by the sun since temperature blistersare likely. Brushing a finish on gives betterpenetration.

PLYWOOD -- FINISHING

AND REFINISHINGExterior plywood is commonly used for siding. For

this application it should have a rough sawn surface(textured) or be overlaid with a stabilized, resin-treated paper called medium density paper overlay(MDO). Smooth sanded plywood is not recom-mended for siding, but it may be used for soffits.Depending upon its intended end use, plywood maybe manufactured from several different woodspecies. Southern pine and Douglas-fir texturedplywood are commonly used in siding applications.The grade of siding used determines the appearanceas well as the wood color. Cedar and redwood are

Figure 1. Plywood (left),particleboard, waferboard and

hardboard (right) aremanufactured or reconstitutedwood products, When painting

or staining, special precautionsshould be taken.

Figure 2. Moisture enters plywoodthrough lathe checks (above)

and early paint failure canresult (below). Paint will flake

from the dense summerwood first.

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Figure 3. Lap marks result with the improperapplicatlon of a penetrating stain.

also commonly used where appearance is impor- penetration and performance especially on texturedtant. Textured-plywood surfaces are probably themost common for exterior siding.

Sanded and roughsawn plywood will developsurface checks, especially when exposed to mois-ture and sunlight. These checks, coupled with theflat grain pattern (wide bands of dark, denselatewood) characteristic of nearly all plywood, canlead to early paint failure (Figure 2). These paintfailures can be minimized by the use of top qualityacrylic latex paint systems.

Semitransparent StainsUnlike paints, semitransparent penetrating oil-

base stains cannot check and peel from plywoodsurfaces. These stains penetrate the wood and donot form a continuous film or coating like paint.Semitransparent penetrating stains allow most ofthe wood grain to show through, and the color canbe controlled by pigments added to the stain. Aproperly chosen stain can accentuate the archi-tectural intent of the textured plywood. Penetratingstains also perform well on weathered surfaces.New, smooth surfaces may also be stained.

Oil-base penetrating stains have a longer lifeexpectancy when properly applied to roughsawn orweathered surfaces. Semitransparent stains may bebrushed or rolled on. Brushing should give better

surfaces. These stains are generally thin and runny,so application can be a little messy. Lap marks willform if stains are improperly applied (Figure 3). Lapmarks can be prevented by staining only a smallnumber of boards or a panel at one time. Thismethod prevents the front edge of the stained areasfrom drying out before a logical stopping place isreached. Working in the shade is desirable becausethe drying rate is slower. The penetrating stainshould be stirred frequently during application. Onegallon will usually cover about 300-400 square feetof smooth surface and from 150-250 square feet ofrough surface.

For long life with penetrating oil-base stain onrough sawn or weathered lumber, use two coats andapply the second coat before the first is dry. Applythe first coat to a panel or area as you would toprevent lap marks. Then work on another area so thefirst coat can soak into the wood for 20 to 60 minutes.Apply the second coat before the first coat has dried.(If the first dries completely, it may seal the woodsurface so that the second coat cannot penetrateinto the wood). About an hour after applying thesecond coat, use a cloth or sponge to wipe off theexcess stain that has not penetrated into the wood.Stain which did not penetrate may form an unsightlysurface film and glossy spots.

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Note: Sponges or cloths that are wet with oil-basestain are particularly susceptible to spontaneouscombustion. To prevent fires, bury them, immersethem in water, or seal them in an airtight containerimmediately after use.

A two coat wet system on rough wood may last aslong as 10 years. If only one coat of penetrating stainis used on new wood, its expected life is 2-4 years,but succeeding coats will last longer.

Refinishing semitransparent penetrating oil-basestains is relatively easy. Excessive scraping andsanding are not required. Simply use a stiff bristlebrush to remove all surface dirt, dust and loose woodfibers, and then apply a new coat of stain. Thesurface should be free of mildew. A longer servicelife can be expected for penetrating stains thesecond time they are applied since they willpenetrate the many small surface checks whichopen up as wood weathers.

Water-Repellent PreservativesWater-repellent preservatives can also be used as

a natural finish for plywood. Water-repellentpreservatives are mixtures of a solvent such asmineral spirits or other paint thinners, wax, a resin ordrying oil and a wood preservative such as penta-chlorophenol or copper naphthenate. These fin-ishes, like semitransparent stains, penetrate thewood and do not form a surface film, so peeling willnot be a problem. Since they do not contain anycoloring pigments, they will allow the natural woodcolor and grain to show through. Expected servicelife is only 1 to 2 years, and frequent reapplication isnecessary to protect the wood surface.

Water-repellent preservatives are best applied bydipping, but brush treatment to the point of refusal isalso satisfactory. It is especially important to applyliberal amounts of the solution to all joints or otherpotential places where moisture might accumulate.Be certain to treat the horizontal bottom edges ofany panels.

Refinishing water-repellent preservatives isaccomplished by simply cleaning the old surfacewith a bristle brush and applying a new coat of finish.To determine if a water-repellent preservative haslost its effectiveness, splash a small quantity of wateragainst the wood. If the water beads up and runs offthe surface, the treatment is still effective. If thewater soaks in, the wood needs to be refinished.Refinishing is also required when the wood surfaceshows signs of graying.

Water repellents are sometimes used in the samemanner as water-repellent preservatives. However,they do not contain a wood preservative and will notprotect against surface mold and mildew.

Note: Steel wool and wire brushes should not beused to clean surfaces which will be finished withsemitransparent stains or water-repellent preserv-atives. Small iron deposits left on the surface canreact with certain wood extratives to form a dark-

blue, unsightly discoloration which is sealedbeneath the new finishing system.

PaintsIn some cases, painting of plywood is required or

desirable. Top quality acrylic latex paints are thebest choice for exterior surfaces. For best perform-ance, use MDO plywood if it is to be painted. Painton MDO will not fail from checking of the wood orfrom peeling because of the wide dark bands ofsummerwood. One primer coat and two top coatsmay last up to 10 years on MDO. Semitransparentpenetrating stains are not effective on MDO.

Refinishing of MDO is usually required only torestore the eroded paint coat. First, remove all loosepaint with a stiff bristle brush and then scrub with asoft brush or sponge and water. Rub your handagainst the cleaned surface to determine if anyresidues remain. When necessary, scrubbing with adetergent or paint cleaner will usually removeadditional residues. Then rinse well and allow to drybefore repainting.

If non-overlaid plywood is to be painted, followthese tips. First, brush a liberal quantity of water-repelient preservative or water repellent onto all theedges of the plywood sheets. The surface shouldalso be treated in the same manner. The waterrepellent will help reduce wood’s tendency toabsorb moisture through the end grain and surfacelathe checks. Allow the water-repellent preservativeor water repellent to dry for at least two warm days.Then prime the plywood surface with a high qualitypaint recommended for use on woods which containextratives. The primer should be applied thickenough to obscure the wood grain pattern. Twocoats of a high quality acrylic latex house paintshould be applied over the primer. Allow at least twodays but no longer than two weeks between theprimer and top coat. The primer and topcoat shouldbe compatible and preferably from the samemanufacturer.

Mildew frequently forms in soffits and on ceilingsof porches and carports. Mildew is common in manyareas, and it is probably best to always use a mildewresistant paint. Always remove the mildew beforerefinishing.

Refinishing painted plywood requires propersurface preparation if the new paint coat is to givethe expected performance. First, scrape away allloose paint. Use sandpaper on any remaining paintto “feather the edges” smooth with the bare wood.Then scrub the remaining paint with a brush orsponge and water. Household bleach (5% sodiumhypochlorite) used at the rate of 1 cup of bleach to 3cups of water will remove mildew. Rinse the surfacewith clean water. Wipe the surface with your hand. Ifthe surface is still dirty or chalky, scrub it again usinga detergent or paint cleaner. Rinse the surfacethoroughly with clean water, and allow it to drybefore repainting. Areas of exposed wood should betreated with a water-repellent preservative or waterrepellent and allowed to dry for at least two days and

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then primed. One or preferably two top coats shouldfollow.

It is particularly important to clean areas pro-tected from sun and rain such as porches and sidewalls protected by overhangs. These areas tend tocollect water soluble materials that interfere withadhesion of the new paint. It is probably adequate torepaint these protected areas every other time thehouse is painted.

RECONSTITUTED WOOD PRODUCTS –FINISHING AND REFINISHING

Reconstituted wood products are those made byforming large sheets, usually 4 by 8 feet, from smallpieces of wood or pulp. Specialized sizes orproducts such as beveled drop siding may also beproduced. Reconstituted wood products may besmooth or textured to look like standard lumber.Depending upon the basic wood component used intheir manufacture, reconstituted wood productsmay be classified as fiberboard or particleboard.

Fiberboards are produced from mechanical pulps.Hardboard is a relatively heavy type of fiberboardand is used for exterior siding. Many grades areavailable.

Particleboard are manufactured from wholewood in the form of flakes, shavings, chips orsplinters. Waferboard and flakeboard are two typesof particleboard made from relatively large flakes orshavings.

Reconstituted wood products can be purchasedunfinished, primed, with a top coat, or stained.Because of the many different types of fiberboardand particleboard products available, most retaillumber dealers do not carry a complete line but canorder any special items.

Only some fiberboards and particleboard aremanufactured for exterior use. Be sure to check withthe supplier, and follow the manufacturer’s direc-tions in using and finishing these board typeproducts. Film-forming finishes such as acrylic latexpaints and solid color acrylic Iatex stains will give themost protection. Other finishes such as semi-transparent stains allow a more rapid deteriorationof the surface to take place.

To paint hard board and particleboard, followgood finishing practices as recommended forplywood. Be sure to seal all edges with a water-repellent preservative or water repellent. Thesurface should be treated with the same solution.Then apply a primer coat recommended for use overwood, followed by at least one high quality top coatof acrylic latex house paint.

Some reconstituted wood products may befactory primed with paint and some may even have atop coat. Factory primed boards should not beallowed to weather for more than a couple of weeksbefore top coating. If excessive weathering doesoccur, clean the factory primed surface, and thenreprime and follow with two top coats.

Reconstituted wood products may be refinishedby following those procedures recommended forplywood.

ADDITIONAL READINGS

Acknowledgement is given to the Forest Products Laboratory, Forest Service, U.S. Department of Agriculture, Madison, Wisconsin forfinancial assistance in support of the preparation of this publication.

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WARNINGDo not mix bleach with ammonia or with any detergents or cleansers containing ammonia!

Mixed together the two are a lethal combination, similar to mustard gas. In several instancespeople have died from breathing the fumes from such a mixture. Many household cleanerscontain ammonia, so be extremely careful with what types of cleaners you mix bleach.

Use caution with wood finishes which contain pesticides. When used improperly they can beinjurious to man, animals and plants. For safe and effective usage, follow the directions, andheed all precautions on the labels. It is advisable to wear unlined protective gloves and to covernearby pIant life when using any material containing pesticides. Do not use any wood finishcontaining pentachlorophenol indoors.

Avoid spraying a pesticide wherever possible. Drift from a pesticide, applied as a spray, maycontaminate the surrounding environment.

Store finishes containing pesticides in original containers under lock and key—out of reachof children and pets — and away from foodstuffs. Follow recommended practices for thedisposal of surplus finishing materials and containers.

Note: Registrations of pesticides are under constant review by the Environmental ProtectionAgency and the Department of Agriculture. Use only pesticides that bears Federal registrationnumber and carry directions for home and garden use. Since the registration of pesticides isunder constant review by State and Federal authorities, you should consult with a responsibleState agency as to the current status of the pesticides discussed in this report.

Sponsored by the Extension services ofIllinois, Indiana, Iowa, Minnesota, Missouri, North Dakota,

South Dakota, and Lincoln Univ, -MO, In cooperation with SEA-Extension, USDA,

Programs and activities of the Cooperative Extension Service are available to allpotential clientele without regard to race, color, sex, national origin, or handicap,

In cooperation with NCR Educational Materials Project.

Issued in furtherance of Cooperative Extension work, Acts of Congress of May 8 and June 30, 1914,in cooperation with the U. S, Department of Agriculture and Cooperative Extension Services of Illinois, Iowa,

Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota and Wisconsin.H. G, Diesslin, Director, Cooperative Extension Service, Purdue University, West Lafayette, Indiana 47907.

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