nafi datafile fm1 (2004) - exterior finishes for timber.pdf
TRANSCRIPT
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Exterior
Finishes forTimber
Revised Edition2004
Timber Manual
Datafle FM1
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NAFITimber
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ExteriorFinishesforTimber
Contents
Introduction......................................................... 2
Factors Effecting Performance.......................... 3
Substrates ............................................................3
Design and Construction .................................... 4
Surface Preparation............................................ 4
Finish Types .........................................................5
Film Forming versus Penetrating Finish......... 5
External nishes .............................................. 7
Permeability ....................................................7
Wood Primers .................................................. 7
Undercoats....................................................... 8
Finishing Coats................................................ 8
Gloss................................................................ 8
Semi-Gloss/Satin............................................. 8
Flat/Matt and Low Sheen ................................ 8
Natural Finishes...............................................9
Semi-Transparent orGrain Enhancing Stains................................... 9
Opaque Stains, Solvent Borne.........................9
Opaque Stains, Water Borne ........................... 9
Clear Exterior Finishes.................................... 9
Application of Coating...................................... 10
Maintenance ...................................................... 11
Specications ..................................................... 11
Introduction
Generally, the function of nishes on timber is two-fold
to improve the durability of the building and to add to
the aesthetics.
Unnished, unprotected timber will inevitably weather
as a result of gradual changes to its physico-chemical
structure brought about by temperature and moisture
content variations. The rate is generally slow, at about
6mm per century.
The weathering process leads to a slow breaking down
and wearing away of surface bres, change in colour and
roughening of the surface. Under extreme conditions,
timber may deform, check, split and pull away from
fasteners. The extent of weathering will vary with timber
species and ambient conditions.
Although unprotected timber has been used externally
for centuries, the weathered effect is not always
desirable. In most applications timber needs protection
from the elements of water, cold, heat and ultra-violet
light to promote a long service life. In addition to
protection from the elements, nishes may provide a
decorative effect.
The performance of exterior nishes depends on a wide
range of factors that are covered in this Datale. The
local environment however, is a variable which needs
special consideration and local experience can be of
assistance. Advice should be sought from manufacturers
or Timber Advisory Services whose contract details are
provided on the back page of this Datale.
The selection and correct application of the most
appropriate nishing product is a most important
component of the design and construction process.This Datale provides guidance on the various
nishes available and the applications for which they
are best suited. It is essential, however, that after a
product has been selected, the product manufacturers
recommendations are followed in respect of timber
preparation, product application and material
maintenance.
Cover Photo: Opaque nish adds colour and
protection to plywood cladding
Weather
exposed
framing
protected and
decorated withopaque nish
blending with
bush setting.
Opaque nished
timber screens
light
colour reects
heat.
This revised edition of Timber Manual Datale FM1 was supported
in part with funding from the Forest and Wood Products Research &
Development Corporation (FWPRDC).
The Corporation is jointly supported by the Australian forest and wood
products industry and the Australian Government.
The information, opinions, advice and recommendations contained
in this Datale have been prepared with due care. They are offered
only for the purpose of providing useful information to assist those
interested in technical matters associated with the specication and
use of timber and timber products. While every effort has been made
to ensure that this Datale is in accordance with current technology,
it is not intended as an exhaustive statement of all relevant data, and
as successful design and construction depends upon numerous factors
outside the scope of the Datale, the National Association of Forest
Industries Ltd accepts no responsibility for errors or omissions from
this Datale, nor for specication or work done or omitted to be done
in reliance on this Datale.
NAFI 1989, 2004
ISBN 1 86346 019 5, ISBN 1 86346 021 7 (set)
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Factors Effecting Performance
The performance of external nishes is a signicant
factor affecting the service life of a building. However,
fashion, new technologies, aesthetic improvements and
changes in function often necessitate modication of
the building, including its external envelope at more
frequent intervals.
In addition to these changes, the durability of an
external nishing system on timber or timber products
is inuenced by a range of physical factors including
those listed in Table 1. Reference should also be made to
Datale P4 -Timber-Design for Durability.
Substrates
The properties of timber, plywood, laminated veneer
lumber, glued laminated timber and other wood based
building products are dealt with in detail in other
Datales in this series.
Timber is essentially a variable substrate, even withina single species. The density, moisture content,
absorbency, exibility and the nature of its extractives
may vary considerably. Some timbers are also more
susceptible to dimensional change due to moisture
or humidity variations. The nishing system must be
sufciently exible to cater for this movement.
Differences in paint performance over earlywood (spring
growth) and latewood (summer growth) can occur
particularly on softwoods where latewood is generally
harder, smoother and darker than earlywood. Paints with
a tendency to embrittle on ageing (that is, those that are
solvent borne) may ultimately crack and peel away from
the harder surface despite having good initial adhesion.
Uniformly ne textured timbers have better paint
holding characteristics than coarse textured species.
The latter, however, respond better to penetrating stain
applications than conventional paint systems.
The manner in which timber is cut from a log affects
the orientation of the annual rings in the piece and this,
in turn, affects its paint holding properties. Back-sawn
softwood boards have wider, darker bands of latewood
than quarter-sawn boards, the latter tending to hold paint
better. In hardwoods the difference is comparatively
small compared to that of softwoods.
If a board is quarter-sawn (edge-grained) as opposed
to back-sawn (at grained), its dimensional stability is
better under variable moisture conditions and it is less
likely to result in surface checking to the detriment of
the paint system.
It should be noted however, that most parcels of timber
are supplied as a mix of back-sawn and quarter-sawn
pieces. Finishing systems are developed to cater for this
variation.
Discolouration or blistering of the nish may be caused
by water-soluble extractives of the heartwood such as
gum or resin. This effect is accentuated by heat and
is more prevalent on the side of a structure receiving
maximum sunlight. Kiln drying of the timber will
usually set these extractives, removing or minimising
the problem.
Some timbers have a higher percentage of aromatic oils,
which contribute to their characteristic pleasant odours
but may present problems with paint discolouration
and inhibit drying if left unprepared before painting.
Cleansing with a solvent may be necessary.
Other timber characteristics that affect paint
performance are knots, bark, gum pockets and insect
damage. Knots will in most cases absorb more of the
nish than side-grain and have a greater potential to
check and exude resin or gum.
Where plywood or glued laminated timber is used, an
exterior timber grade must be specied together with aconventional nishing system for timber.
Exterior grade hardboard has a smooth uniform surface
nish and is usually supplied pre-primed, ready for
nishing.
Table 1: Factors effecting performance of nishes
Semi-
transparent
timber stains
enhance
resorts
residential
complex
Factor Effect on Performance
Substrate Refer Table 2.
Environmental
Conditions
Dry temperate conditions favour very
good performance compared with
tropical, marine and severe industrial
environments which provide
relatively severe exposure conditions.
Pre-treatment Use of preservative impregnation or
water repellent preservative treatment
before painting, together with putties
and sealants where appropriate, will
improve service life.
Maintenance
Schedule
Re-coating before major paint
lm breakdown occurs, results in
improved durability at reduced cost.
Application
Procedure
Adequate lm build and attention to
coating end-grain results in improved
performance.
Building
Design
Protection from elements using roof
overhangs, verandahs, pergolas,
etc., and avoiding water traps will
promote improved performance.
System choice Correct choice of a system for
each particular job and use of one
manufacturers products in each
system will contribute to the long
term performance of the nish.
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Design and Construction
The two major factors which affect the performance
of the nish, and subsequently that of the timber, are
moisture and sunlight. Any design features which
minimise possible moisture ingress to the substrate
or exposure to high angle incident sunlight, will be
benecial.
In Australia (Southern Hemisphere) the north facing
exposure is most severely affected by sunlight, with the
severity of exposure changing with the latitude of the
site itself, varying from 15 degrees near Cape York to
about 45 degrees near Tasmania. Guidance on the design
of structures to provide shade protection is given in NSB
120, Shadow Angles.
Some design steps which can be taken to assist in
excluding moisture and protecting from U.V. are:
the use of wide eave overhangs, verandas, pergolas
etc.
use of vertical rather than horizontal siding
use proles with rounded arrises t drip caps over doors and windows
Avoid or minimise joins in horizontal sidings likely
to allow moisture ingress
provide adequate ventilation of roof space and sub-
oor space
use the recommended nail size and pattern for
various types of timber cladding
selectively use boards that show defects, such as
knots. When you do, use them for the least exposed
areas.
Surface Preparation
The purpose of preparing a substrate for painting is to
ensure that it will accept and retain the nish with the
minimum of interference from surface contaminants or
from surface deterioration of the substrate itself. The
prole of the surface and its porosity will also inuence
adhesion of the paint system and its ultimate durability.
For new timber, the prole can vary from a smooth
(planed) surface to a sawn textured surface. This will
Substrate
Variable
Effect on Performance
Species Differs between and within species.
Density The heavier, denser timbers are lessprone to moisture uptake than lighter
species
Dressed timber Better performance with conventionalpaint systems.
Sawn timber Better performance with solid andnatural stains.
Timber edges Sharp edges create stress in paint lms,causing failure. Arrised or rounded
edges permit lm movement with
minimal stress.
Back-sawn
timber versus
Quarter-sawn
timber
Quarter-sawn softwood boards
generally have better dimensional
stability than back-sawn, and therefore
better paint holding properties. For
hardwood the difference is minimal.
Unseasoned
(green) timber
Greater likelihood of blistering and
peeling occurring particularly with
solvent borne coatings. Stains are
more effective.
Seasoned timber Best condition for painting.
Weathered
versus
unweathered
timber
Surface deterioration of timber bres
due to weather can result in poor paint
adhesion unless corrective sanding is
carried out. Stains may perform better
than conventional nishes if sanding is
omitted.
Extractives May cause topcoat discolouration or
blistering unless surface extractives areremoved with a solvent wash prior to
priming.
Heartwood
versus Sapwood
Heartwood has a higher staining
potential than sapwood but is ideally
suited to painting with exterior timber
stains.
Earlywood
versus Latewood
Cracking on latewood is more likely
to occur as the solvent borne paint
embrittles on extended exposure.
Smooth texture
versus coarse
texture
Smooth texture surfaces have better
paint holding potential than coarse
texture ones. Use of stains on the
latter gives good performance.Gum pockets Can lead to resin exudation and
staining unless pre-treated and sealed.
Aromatic oils Can lead to drying retardationand staining if surface oils are not
removed.
Knots Possible premature cracking, stainingand resin exudation can occur unless
treated with knotting varnish or
manufacturers recommend treatment.
Bark Can lead to premature failure of alllm-forming nishes if not removed.
Insect Damage Premature failure may occur if nottreated or lled before nishing.
Moisture
Content
Normal range 10-15% - higher levels
detrimental, particularly for solvent
borne nishes where moisture vapour
permeability is low.
Table 2: Substrate effect on performance
Knots and sawn
surface texture
show through
semi-transparent
timber stain.
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dictate the nal appearance when coated.
The sawn or roughened (coarse sand papered) surface
will give a more positive mechanical key to applied
coatings but, in the case of a conventional paint system,
will result in a much thinner lm on the surface peaks
which will deteriorate more rapidly. Some surfaces are,
however, ideally suited to opaque or semi-transparent
stains.
Timber with excessively high moisture or resin/aromatic
oil content should undergo further seasoning to rectify
the conditions before painting.
As recommended by the paint manufacturers,
surface contamination by dust, grease, oil, plaster or
mortar droppings, brick-cleaning solutions, or other
substances likely to affect paint adhesion, must be
thoroughly cleaned before priming. Prevention of such
contamination is better than subsequent cleaning.
A special consideration exists with buildings adjacent to
the sea, where salt contamination of the surface occurs
very rapidly and is not always visible. This must be
washed off with fresh water shortly before the start of
work, while still allowing time for the surface to dry. If
prevailing winds are carrying salt laden air to the site,
repeat washings will be required each day painting is to
proceed.
For timber which has been exposed to the elements
for a considerable time and has suffered the weathers
oxidative effects on both lignin and cellulose in the
surface layer, the timber should ideally be sanded or
dressed to a fresh surface before priming. Alternatively
the use of penetrating type stains, after a simple brush
down, will give good results.
Any holes or depressions in the surface which mayhave occurred as a result of mechanical damage or are
natural defects such as resin pockets, knots, surface
splits or checks should be scraped clean, primed and
lled with putty compatible with the paint system to be
used. Damaged or decayed sections should be cut out
and replaced. Adjacent timber should be treated with
a fungicide or water repellent solution. The source of
moisture giving rise to conditions conducive to decay
should also be located and the fault rectied.
As with design, emphasis must be placed on preventing
moisture ingress, particularly where end-grain is
concerned. Adequate sealing, with sealants, water
repellents or primers is essential.
Some timbers and wood based building boards are
supplied pre-primed and provided they are not exposed
to the elements for extended periods, may be nished
with undercoat and an enamel paint or two coats
of an acrylic paint without further priming, (or as
recommended by the manufacturer).
Veranda roof
provides
protection to
external walls.
The preparation for repainting a previously painted
surface will depend largely on the condition of the
surface. If the paintwork is aking, blistering, cracking
or checking, it will be necessary to remove either the
entire lm back to the substrate or at least enough of it
to provide a sound base for subsequent coats.
If the surface is chalking, or has surface mould present,
manufacturers directions for removal should be
followed. This is particularly critical where water borne
systems are being used.
Finish Types
There is a wide range of nish types with selection
being governed by the degree of protection required and
the appearance sought. Table 3 summarises the various
nishes.
Table 4 provides guidance on the selection of nishing
systems for various applications, but advice is also
available from manufacturers and reference can be made
to the standard AS/NZS 2311 Guide to the Painting of
Buildings.
Film Forming versus Penetrating Finish
All nishes provide a coating on the surface and to some
extent ll voids in the microscopic surface structure.
Film forming and penetrating nishes vary in thickness
and in the nished shape of the surface.
Film forming nishes appear as a distinct layer and
usually display a plane surface. Penetrating nishes on
the other hand follow the contours of the timber surface,
providing thicker deposits in the troughs and thinner
coatings over the peaks. Refer to Figure 1.Due to the greater coating thickness, lm forming
nishes are more resistant to wear, but penetrating
nishes have the ability to accommodate differential
movement of timber due to moisture variations. The
Water repellent-
containing
preservatives
can provide
protection from
moisture but
require regular
maintenance.
Sawn nished
plywood provides
a good base for
surface nishing.
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Paint Type Comments
Conventional opaque systems -
water and solvent borne
Best suited for use on dressed and seasoned timber. Wide colour ranges with
use of tinting systems, good durability.
Natural timber nishes - semi-
transparent and opaque
Best suited for use on sawn or textured durable timbers. More frequent but
much simpler maintenance.
Water Repellent Preservatives Temporary pre-treatment to prevent soiling and deterioration during
construction but have limited life if not overcoated.
Wood Primers Brush application is recommended. Solvent borne better penetration,
but slower drying. Water borne better long term exibility, quicker
overcoating and easier clean up.
Undercoats Good opacity and bridging properties. Increase system lm build and
durability. Solvent borne best over chalky surfaces. Water borne faster
drying, better colour qualities and easier clean up.
Finishing Coats - Gloss Solvent borne have sharper gloss, good ow, opacity and durability. Water
borne have slightly lower initial gloss, better gloss retention, poorer ow
but better lm exibility after aging. Water borne system applied to primed
timber should prove superior durability to solvent borne.
Finishing Coats - Semi-Gloss Comments as for gloss but gloss level lowered. Slight reduction in durability
but minimise appearance of surface imperfections.
Finishing Coats - Flat and Low
Sheen
Solvent borne rarely used for exterior. Water borne good durability and
colour.
Natural Finishes - Opaque or Solid
Stains
Generally solvent borne. Enhance both timber grain and texture and help
maintain timber colour. Care required in application to avoid lap joint colour
difference.
Natural Finishes - Semi-
Transparent Stains
Obscures grain but enhances texture. Have better durability than semi-
transparent stains. Water borne acrylic solid stains generally give superior
performance to solvent borne nishes particularly over knotty timbers.
Natural Finishes - Clear Enhances timber appearance but requires a commitment to more regular
maintenance with frequent inspections. Solvent borne clears have improved
with use of U.V. absorbers specic to resin type used. Water borne clears,or slightly tinted versions, have durability somewhat equivalent to opaque
systems. Water borne clears are more exible and thermoplastic.
Table 3: Exterior nishes and applications
Finishing Systen Cladding Decking Joinery Framing
H
ardwoodand
C
ypressPine
S
oftwood
H
ardwoodand
C
ypressPine
S
oftwood
H
ardwoodand
C
ypressPine
S
oftwood
H
ardwoodand
C
ypressPine
S
oftwood
1. Primer + undercoat + minimum 2
coats solvent borne or water borne
paint
2. Primer + minimum 2 coats solvent
borne or water borne opaque stain
3. WRP + 2 coats solvent borne stain - - - -
4. Minimum 2 coats solvent borne
or water borne opaque or semi-
transparent
- - - -
5. Minimum 3 coats clear exteriornish
- -
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Table 4: Finishing Recommendations
Notes:
1. This table is provided as a guide only to those nishing systems suitable for the specied applications.
2. Stains are more successful when applied to sawn nishes.
3. Clear exterior nishes have a relatively short service life and require regular maintenance when used in a fully exposed situation.
.
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advantages of penetrating nishes include:
natural appearance
do not peel or blister
suited to sawn textured surfaces
do not trap moisture in timber
are easily applied and renewed
External nishes
Many pigmented nishing systems for both protection
and decoration consist of a three-stage process:
primer
undercoat
If a nish is not applied on all surfaces of a piece
of timber, differential moisture changes may cause
distortion. Such problems may be overcome by
providing balanced moisture permeable coatings on all
surfaces.
Finishing coats and the nishes used for external
applications can be divided into two broad categories:
solvent borne
water borne
Permeability
Most coatings are impervious to water but none are
completely impervious to water vapour. Surface lms
are superior in this regard but after initial cracking or
peeling, protection is dependent on the nish that has
penetrated into the wood. Penetrating nishes may
therefore be more effective.
Solvent borne nishes are more resistant to water vapour
than water borne nishes, and where a high level ofprotection is desired, a solvent borne system should be
selected. Where recommended by the manufacturer, a
solvent borne primer and/or undercoat system may be
used under a water borne nishing system to provide
Figure 1: Finish types
improved water vapour protection.
Wood Primers
Wood primers provide good adhesion to the timber and a
good base for inter-coat adhesion of subsequent coats.
They may be solvent borne, comprised of drying oils
and/or synthetic alkyd resins and can incorporate zinc
oxide or other fungicidal pigment to assist in fungal
control of the system.
The more recently developed, water borne primers,
containing an acrylic emulsion as the principal binder,
are an alternative which offers good adhesion to
timber and greater exibility, particularly on ageing.
The problem of staining, when applied to timbers
containing natural, water soluble tannins, has been
largely overcome by use of xative pigments and special
emulsions developed for this purpose. Water borne
systems allow greater water vapour permeability than
solvent borne systems.
Current wood primers are lead free and are generally
tinted to a pink colour. This provides greater apparent
opacity and a contrasting colour to the next coat being
applied. It avoids areas being missed during application
and ensures that a sufcient second coat is applied to
adequately cover the primer uniformly.
Note that timber treated with light organic solvent
preservatives (refer AS 1604 Specication for
preservative treatment) may be delivered in a pre-
primed condition. This should not be mistaken for
properly primed timber ready for top coating. Pre-
primers are low cost, solvent-borne, quick drying,
factory applied products, which are not formulated to
provide long service life and exibility of nish.
Undercoats
These are the bridging coats between the primer or
previous paint surface and the nishing coat. They have
Weather exposed
structure and
joinery to have
ends and joints
sealed against
water penetration.
Primingprovides
protection to
timber and a
good base for
nishing coats.
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good bridging properties across cracks, good brushability
under warm windy conditions, facilitate sanding, good
gloss hold out, enhance the durability of the total system
and have good opacity that is capable of substantially
obscuring the background colour or colours.
Similar to primers, they can be solvent or water borne.
Solvent borne undercoats are generally superior for
adhesion if applied to slightly chalky or powdery surfacesand can be formulated to give superior sanding properties.
Water borne undercoats have better colour and non-
yellowing qualities. They are faster drying with much
shorter times before overcoating. They may be over-
coated with either water or solvent borne coatings, subject
to manufacturers recommendations.
Finishing Coats
In all pigmented coating systems, (better known as paints,
as distinct from stains), the water borne paints have a
decided advantage over solvent borne paints in terms of
ease of clean up of equipment and faster drying and re-coat times. One restriction, however, is that they must not
be applied or allowed to dry at temperatures below 10
degrees Celsius, as lm formation may not occur and the
paint may remain powdery.
Manufacturers directions should be followed at all times,
particularly with regard to the system or succession of
coats to be applied.
Gloss
These are available in a very wide range of colours
in both solvent and water borne coatings. The need to
provide a wide colour range, while restricting stock items,has led to the use of tinting systems in preference to large
numbers of ready mixed colours.
Most solvent thinned gloss enamels can be used for
interior as well as exterior use, but certain colours may
be restricted by the manufacturer to interior use only
because of the poor light fastness of certain colours. Most
solvent borne exterior enamels are formulated with alkyd
resins and have largely replaced the traditional linseed oil
house paints. They also contain small amounts of acrylic
polymers to provide the nish with some exibility. They
have a sharp gloss, good opacity and exterior durability.
Water borne gloss exterior nishes are now widely used.
They have superior chalk resistance, exibility and gloss
retention. They frequently provide a useful lm life of
double that of the solvent systems, if they are applied
to suitably prepared primed surfaces. When these water
Quality nishes
enhance
appearance and
promote long
service life.
borne nishes are used for re-painting it should be
remembered that old, embrittled alkyd or oil-based
nishes will continue to embrittle with time and will
eventually fail irrespective of the type of re-paint top
coat used.
Exterior water borne nishes are generally based on
emulsion resins, the most popular and proven type
being pure or 100% acrylic emulsions.
Semi-Gloss/SatinSatin and semi-gloss nishes, for both solvent and
water borne coatings, are similar in many respects to
their full gloss counterparts but have had the gloss level
reduced by the addition of attening agents.
While quite suitable for exterior use, when
recommended by the manufacturer, they tend to chalk
earlier but do not highlight minor imperfections in
the substrate as readily as gloss nishes. They are
frequently available in the same extensive colour range
as gloss nishes but have an increased number of
colours that are suitable only for interior use.
Solvent borne semi-gloss nishes for exterior use are
alkyd based, while water borne coatings are mainly
100% acrylic based, to give maximum exterior
durability.
Flat/Matt and Low Sheen
In this category the use of solvent borne nishes for
exterior use is negligible and water borne nishes
are almost universally preferred. The use of 100%
acrylic emulsions gives very good exterior exposure
performance over primed timber substrates where lm
exibility on ageing is important.The term low sheen is generally used to indicate a
nish with a slightly higher sheen than either at
or matt. It is more washable and still masks minor
imperfections in the substrate more satisfactorily than
gloss or semi-gloss nishes.
Natural Finishes
Semi-transparent and opaque or solid stains have been
developed to provide a more natural timber-look to a
surface and to simplify maintenance procedures.
The natural nishes are designed primarily for use on
sawn timbers. Normally a one or two coat applicationof stain does not result in the thicker coating achieved
with conventional paint systems, and therefore there
is little risk of breakdown of the lm by cracking and
aking. These stain nishes weather away by natural
erosion, allowing subsequent re-coating with a minium
of preparation.
A simple brush and wash down to remove dirt and
chalking pigment is all that is required. The periods
between re-coating are generally shorter than with
conventual systems, but tend to increase with
subsequent re-paints.
The build-up or thickness of lm of these natural
nishes will vary from product to product. At the
extremes it includes coatings which penetrate into
the timber leaving little or no lm on the surface, to
others which resemble top coat paints. These have
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Care must be taken in using products of this type to
avoid lap marks, as these clear exterior nishes dry more
rapidly than an oil stain and a double application at the
lap gives rise to bands of deeper colour.
Application of Coating
The most common application technique for exterior
timber nishing is by brush, which although morelabour intensive, is specically recommended for initial
priming coats as it gives better wetting of the surface
and penetration into grain, joints and corners. Roller
application or airless spray may be used for subsequent
exterior coats and is more suited to large areas.
The effectiveness of an exterior paint system is
dependent on building a lm of adequate thickness and
overspreading of coatings is a false economy. At all
times the manufacturers recommendations, with regard
to thinning, surface preparation, number and sequence of
coats, maximum spreading rates, weather conditions and
temperature at times of application and drying, and timeinterval between coats, should be adhered to.
Apart from minimum times between coats, it is
advisable that no longer than 2 weeks should elapse
between coats without attention being paid to cleaning
the surface to remove any grime or dust that may have
built-up. Conditions at the site will obviously inuence
the extent of this contamination. If the site is subject to
salt contamination from prevailing winds off the sea,
then washing down daily may be necessary.
The application of stains, particularly semi-transparent
types, should be in the direction of the wood grain
and completed in one operation if lap marks are to beavoided.
Factory priming or treatment with water repellent-
containing preservative of some seasoned timber
components, such as window assemblies, is designed to
give them protection from weather and contamination
during transport, site storage and an initial period after
installation. But, as already mentioned, such factory pre-
primed timber should not be considered as equivalent to
properly primed timber.
Joinery, cladding and other timber building materials
delivered to the site unpainted should be primed all
around as soon as practicable. Any cutting or handlingthat removes primer or leaves uncoated end-grain should
be primed before nal xing.
Likewise any primed work that has been allowed to
deteriorate through exposure, should be cleaned down
and re-primed.
Exterior hardboard wall claddings are usually supplied
factory primed. Some unprimed boards do not require
priming when timber coloured, 100% acrylic nishes
are used. The unprimed and primed boards should
normally be paint nished within 60 days after xing, as
recommended by the manufacturer.
It is important that any surface to be painted, whether
previously painted or not, should be suitably prepared to
receive paint.
Painting should not be undertaken during extremely hot
weather or when temperatures fall below 10 degrees
Celsius. Foggy, misty or dusty weather should also
be avoided. Work for the day should be stopped early
enough to allow for the lm to dry sufciently before
adverse conditions develop.
Maintenance
Because of the wide variation in the severity of exposure
conditions, the variability of timber substrates, thecare taken in the initial priming and nishing, and the
number of systems applied over a period of years, it is
impossible to predict the service life of a paint system
before complete re-coating is necessary. Although on
average, a period of 5 to 10 years may be expected, it
is more likely that specic areas, such as those adjacent
to timber end-grain, window sills facing north, or sharp
arrised sections, may need remedial coating before this
time. In order to minimise such action, it is advisable to
give those areas an additional coat, either primer or top
coat at the time of initial painting.
The opposite situation arises on areas which are wellprotected, such as under wide eaves or verandas,
which may not need re-coating on each repaint
occasion, particularly as too great a build-up of paint
will ultimately require major maintenance involving
complete stripping before coating again.
While excessive paint lm build up is to be discouraged,
it is nevertheless most important to re-coat before
general deterioration occurs.
The presence of mould and mildew requires removal
with fungicidal washes, and rinsing and drying before
subsequent coats are applied. If this is not done, mould
can spread between coats of paint and result in lifting ofthe paint lm and discolouration.
Stains require more frequent maintenance but this is
generally much easier to carry out both in terms of
surface preparation and in coating application.
Reference should also be made toDatale P4 -Timber-
design for durability.
The presence of mould and mildew requires removal
with fungicidal washes, and rinsing and drying before
subsequent coats are applied. If this is not done, mould
can spread between coats of paint and result in lifting of
the paint lm and discolouration.
Stains require more frequent maintenance but this is
generally much easier to carry out both in terms of
surface preparation and in coating application.
Reference should also be made toDatale P4 -Timber-
design for durability.
White
opaque nish
protectstimber and
contrasts with
dark nished
sun-protected
walls.
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