Exterior Finishes for Wood
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All too often, exterior finishes for wood are
short lived in durability or fail completely. Failure is
usually the result of the wrong kind of finish being
applied to the wood surface or of not following
recommended application procedures. Instruction following
is given in selecting the appropriate finish/wood
combination and in following through by applying the
finish as specified by the manufacturer.
Types of Wood Products
Each product has unique characteristics which will
affect the durability of any finish applied to it.
Lumber-dimensional change in lumber occurs as the wood
gains or loses moisture. Wood in heated homes tends to
dry and shrink in the winter and gains moisture and
swells in the warm summer months. Excessive dimensional
change in wood consistently stresses a paint film and may
result in early failure.
Grain direction affects paint-holding characteristics
and is determined at the time lumber is cut. Edge-grained
bevel siding will hold paint well. Flat-grained lumber
will not hold paint as well since it shrinks and swells
more than edge-grained lumber and because wide, dark
bands of summerwood are frequently present.
Paint will last longer on smooth, edge-grained
surfaces. Penetrating stains or preservative treatments
are preferred for rough sawn lumber. These treatments
often accentuate the natural or rustic look of rough sawn
lumber and allow the wood grain and surface texture to
show through the finish.
Plywood
Sanded and rough sawn plywood will develop surface
checks, especially when exposed to moisture and sunlight.
These surface checks can lead to early paint failure with
oil or alkyd paint systems.
Plywood manufactured with a medium density paper
overlay is frequently called MDO. Compared to standard
rough sawn or smooth plywood, MDO holds paint well. MDO
plywood is not always a stock item in many lumber yards,
but it can usually be ordered.
Types of Wood Finishes
Paint
Paints are common coatings used on wood and provide
the most protection. Latex-based paints and stains are
water borne, and oil or alkyd paints are solvent borne.
Paints are used for aesthetic purposes, to protect the
wood surface from weathering and to conceal certain
defects.
Paints are applied to the wood surface and do not
penetrate it deeply. The wood grain is completely
obscured, and a surface film is formed. This surface film
can blister or peel if the wood is wetted or if inside
water vapor moves through the house wall and wood siding
because of the absence of a vapor barrier.
Latex paints are generally easier to use since water
is used in clean-up. They are also porous and, thus, will
allow some moisture movement. In comparison, oil-based
paints require organic solvents for clean-up, and some
are resistant to moisture movement. Paints are the only
way to achieve a bright white finish. However, they are
subject to peeling where moisture is a problem.
Solid Color Stains
Solid color stains, also called heavy bodied stains,
are opaque finishes which come in a wide range of colors
and are defined as stains. Solid color stains are made
with a much higher concentration of pigment than the
semitransparent penetrating stains. As a result they will
obscure the natural wood color and grain. Oil-based solid
color stains tend to form a film much like paint and as a
result can also peel loose from the substrate. Latex-
based solid color stains are also available and form a
film as do the oil-based solid color stains. These stains
are similar to thinned paints.
Semitransparent Penetrating Stains
They are moderately pigmented and, thus, do not
totally hide the wood grain. These stains penetrate the
wood surface, are porous, and do not form a surface film
like paints. As a result, they will not blister or peel
even if moisture gets into the wood. Penetrating stains
are alkyd or oil-based, and some may contain a fungicide
or water repellent. Latex-based (water borne) stains are
also available, but they do not penetrate the wood
surface as do the oil-based stains.
Stains are most effective on rough lumber or plywood
surfaces. They are available in a variety of colors and
are especially popular in the brown tones since they give
a "natural or rustic wood appearance." They are not
available in white. They are also an excellent finish for
weathered wood. They are not effective when applied over
a solid color stain or over old paint.
Water-Repellent Preservatives
A water-repellent preservative may be used as a
natural finish for wood surfaces. It contains a wood
preservative, a small amount of wax as a water repellent,
a resin or drying oil, and a solvent such as turpentine
or mineral spirits. Water-repellent preservatives do not
contain any coloring pigments. Therefore, the resulting
finish will vary in color depending upon the kind of
wood. The preservative prevents wood from graying by
inhibiting mildew.
Water-repellent preservatives may also be used as a
treatment for bare wood before priming and painting or in
areas where old paint has peeled, exposing bare wood.
This treatment keeps rain or dew from penetrating into
the wood, especially at joints and end grain, and thus
decreases the shrinking and swelling of wood. As a
result, less stress is placed on the paint film, and its
service life is extended. This stability is achieved by
the small amount of wax present in water-repellent
preservatives. The fungicide inhibits surface decay.
Be sure to purchase the correct type of water-
repellent preservative. Any type of water-repellent
preservative can be used as a natural exterior finish by
itself, but only some are paint able. Manufacturers have
also developed water-repellent preservatives specifically
for exterior finishes.
Water repellents are also available. These are
simply water-repellent preservatives with the
preservative left out. Water repellents are not good
natural finishes but can be used as a stabilizing
treatment before priming and painting.
Before purchasing and using a water-repellent
preservative or water repellent, read the label carefully
and follow the manufacturer's directions.
Varnishes
Varnishes, synthetic resins and other clear film-
forming finishes provide an attractive finish for wood
since they allow the natural wood color and grain to show
through. Unfortunately, the durability of these finishes
on wood under the action of sunlight and moisture is
limited. Regardless of the number of coats applied, the
film will begin to crack and peel, and the finish will
have to be completely removed by sanding or with a
varnish remover before a new coat is added. Alternate
finishes such as semitransparent stains and water-
repellent preservatives will give a longer service life
and are easier to refinish.
Exterior marine or spar varnishes may be used with
some success on exterior doors and other areas if
adequate protection from the weather is provided.
Preservatives
Wood preservatives are not considered to be finishes.
However, wood properly treated with a preservative can
withstand years of exposure to severe decay and insect
attack without being affected. The common wood
preservatives are creosote, penta-chlorophenol in oil,
and the newer water-borne salt treatments---all of which
are restricted-use pesticides. Creosote and
pentachlorophenol in oil result in a dark and oily
surface. Odor with creosote is a problem. Wood treated
with creosote or pentachlorophenol in oil is not
recommended for use around the home where people will
come in contact with it. However, wood treated with
water-borne salts is suggested for use as patio decks,
outside steps, privacy fences and other home uses. This
material is generally light to bright green or brown in
color. It can be used outdoors without finishing and will
go practically unchanged or weather to a light gray.
Application of Wood Finishes
Paint
Proper surface care and preparation before applying
paint to wood is essential for good performance. Wood and
wood-based products should be protected from the weather
and wetting on the job site and after they are installed.
Surface contamination from dirt, oil and other foreign
substances must be eliminated. It is best to paint wood
surfaces within 2 weeks, weather permitting, after
installation.
To achieve maximum paint life, follow these steps:
1. Wood siding and trim should be treated with a paintab
water-repellent preservative or water repellent. Water
repellents protect the wood against the entrance of rain
and dew and thus prevent swelling and shrinking. They can
be applied by brushing or dipping.
Allow at least two warm, sunny days for adequate
drying before painting the treated surface. If the wood
has been dip-treated, allow at least one week of
favorable weather.
2. After the water-repellent preservative or water
repellent has dried, the bare wood must be primed. Since
the primer coat forms a base for all succeeding paint
coats, it is very important. For woods with water-soluble
extractives such as redwood and cedar, the best primers
are good quality oil-based and alkyd-based paints. Some
latex-based primer paints are also designed for use over
these woods. The primer seals in the extractives so that
they will not bleed through the top coat. A primer should
be used whether the top coat is an oil-base or latex-base
paint. For species which are predominately sapwood and
free of extractives, such as pine, a high quality acrylic
latex paint may be used as both a primer and top coat.
Enough primer should be applied to obscure the wood
grain. Do not spread the primer too thinly. Follow the
spreading rates recommended by the manufacturer. A primer
coat which is uniform and of the proper thickness will
distribute the swelling stresses which develop in wood
and thus prevent premature paint failure.
3. Two coats of a good-quality acrylic latex house paint
should be applied over the primer. If it is not practical
to apply two top coats to the entire house, consider two
top coats for fully exposed areas on the south and west
sides.
Areas fully exposed to sunshine and rain are the
first to deteriorate and therefore should receive two
coats. On those wood surfaces best suited for painting,
one coat of a good house paint over a properly applied
primer (a conventional two-coat paint system) should last
4 to 5 years, but two coats can last up to 10 years.
4. One gallon of paint will cover about 400 square feet
surface area. However, coverage can vary with different
paints and application procedures. Research has indicated
that the optimum thickness for the total paint coat
(primer and two top coats) is 4-5 mils or about the
thickness of a sheet of newspaper. The quality of paint
is usually, but not always, related to the price. Brush
application is always superior to roller application.
Additional tips on painting
To avoid future separation between paint coats, the
first top coat should be applied within 2 weeks after the
primer and the second coat within 2 weeks of the first.
As certain paints weather they can form a soap-like
substance on their surface which may prevent proper
adhesion of new paint coats. If more than 2 weeks elapse
before applying another paint coat, scrub the old surface
with water using a bristle brush or sponge. If necessary,
to remove all dirt and deteriorated paint, use a mild
detergent. Then rinse well with water, and allow the
surface to dry before painting.
To avoid temperature blistering, oil-base paints
should not be applied on a cool surface that will be
heated by the sun within a few hours. Temperature
blistering is most common with thick paint coats of dark
colors applied in cool weather. The blisters usually show
up in the last coat of paint and occur within a few hours
to 1 or 2 days after painting. They do not contain water.
Oil-base paint may be applied when the temperature is
40 degrees F or above. A minimum of 50 degrees F is
desired for applying latex-based paints. For proper
curing of the paint film, the temperature should not drop
below 50 degrees F for at least 2 hours after paint
application. Low temperatures will result in paint
failure.
To avoid wrinkling, fading or loss of gloss of oil-
base paints and streaking of latex paints, the paint
should not be applied in the evenings of cool spring and
fall days when heavy dews form before the surface of the
paint has thoroughly dried.
Solid Color Stains
Solid color stains may be applied to a smooth surface
by brush or roller application, but brush application is
best. These stains act much like paint. One coat of solid
color stain is adequate, but two coats will provide
better protection and longer service.
Unlike paint, lap marks may form with a solid color
stain. Latex-based stains are fast-drying and are more
likely to show lap marks than the oil-based ones. To
prevent lap marks follow the procedures suggested under
application of semitransparent penetrating stains.
Semitransparent Penetrating Stains
Semitransparent penetrating stains may be brushed or
rolled on. Brushing will give better penetration and
performance. These stains are generally thin and runny,
so application can be messy. Lap marks will form if
stains are improperly applied. Lap marks can be prevented
by staining only a small number of boards or a panel at
one time. This method prevents the front edge of the
stained area from drying out before a logical stopping
place is reached. Working in the shade is desirable
because the drying rate is slower. One gallon will
usually cover about 300-400 square feet of smooth surface
and from 150-200 square feet of rough surface.
For long life with penetrating oil-base stain on
rough sawn or weathered lumber, use two coats and apply
the second coat before the first is dry. Apply the first
coat to a panel or area as you would to prevent lap
marks. Then work on another area so that the first coat
can soak into the wood for 20 to 60 minutes. Apply the
second coat before the first coat has dried. (If the
first coat dries completely, it will seal the wood
surface so that the second coat cannot penetrate into the
wood.) About an hour after applying the second coat, use
a cloth or sponge to wipe off the excess stain that has
not penetrated into the wood. Stain which did not
penetrate will form an unsightly surface film and glossy
spots. Avoid intermixing different brands or batches of
stain. Stir stain thoroughly during application. Note.-
Sponges or cloths that are wet with oil-base stain are
particularly susceptible to spontaneous combustion. To
prevent fires, bury them, immerse them in water, or seal
them in an airtight container immediately after use.
A two-coat wet system on rough wood may last as long
as 10 years in certain exposures. If only one coat of
penetrating stain is used on new wood, its expected life
is 2 to 4 years, but succeeding coats will last longer.
Water-Repellent Preservatives
The most effective method of applying a water-
repellent preservative is to dip the entire board into
the solution. However, brush treatment is also effective.
When wood is treated in place, liberal amounts of the
solution should be applied to all lap and butt joints,
edges and ends of boards and panels. It is important to
apply liberal amounts of the solution to the end grain of
wood. Areas especially vulnerable to moisture, such as
the bottoms of doors and window frames, should not be
overlooked. One gallon will cover about 250 square feet
of smooth surface or 150 square feet of rough surface.
The life expectancy is only 1-2 years, depending upon the
wood and exposure. Treatments on rough surfaces are
generally longer-lived than those on smooth surfaces.
Repeated brush treatment to the point of refusal will
enhance durability and performance.
Refinishing
Paint
If you are refinishing an old paint coat, proper
surface preparation is essential if the new coat is to
give the expected performance. First, scrape away all
loose paint. Use sandpaper on any remaining paint to
"feather" the edges smooth with the bare wood. Then scrub
any remaining old paint with a brush or sponge and water.
Rinse the scrubbed surface with clean water. Wipe the
surface with your hand. If the surface is still dirty or
chalky, scrub it again using a detergent. Mildew should
be removed with a dilute household bleach solution. Rinse
the cleaned surface thoroughly with fresh water and allow
it to dry before repainting. Areas of exposed wood should
be treated with a water-repellent preservative, or water
repellent, and allowed to dry for at least two days, and
then primed. Top coats can then be applied.
Latex paint can be applied over freshly primed
surfaces and on some surfaces where an oil-base paint has
already been used and weathered. Where old surfaces are
to be repainted with latex paint a simple test should be
conducted first. After cleaning the surface, repaint a
small, inconspicuous area with latex paint, and allow it
to dry at least overnight. Then, to test for adhesion,
firmly press one end of a "band aid" type adhesive
bandage onto the painted surface. Jerk it off with a
snapping action. If the tape is free of paint, it tells
you that the latex paint is well bonded and that the old
surface does not need priming or additional cleaning. If
the new latex paint adheres to the tape, the old surface
is too chalky and needs more cleaning or the use of an
oil-base primer.
Solid Color Stains
The same technique used to restore a paint coat can
be used for solid color stains.
Semitransparent Penetrating Stains
Semitransparent penetrating stains are relatively
easy to refinish. Excessive scraping and sanding are not
required. Simply use a stiff bristle brush to remove all
surface dirt, dust, and loose wood fibers, and then apply
a new coat of stain. The second coat of penetrating stain
often lasts longer since it penetrates into small surface
checks which open up as wood weathers.
Water-Repellent Preservatives
Water-repellent preservatives can be renewed by a
simple cleaning of the old surface with a bristle brush
and an application of a new coat of finish. To determine
if a water-repellent preservative has lost its
effectiveness, splash a small quantity of water against
the wood surface. If the water beads up and runs off the
surface, the treatment is still effective. If the water
soaks in, the wood needs to be refinished. Refinishing is
also required when the wood surface shows signs of
graying.
Note.- Steel wool and wire brushes should not be used
to clean surfaces to be finished with semitransparent
stains or water-repellent preservatives since small iron
deposits may be left behind. Pentachlorophenol may cause
iron remaining on the surface to corrode. The corrosion
products may then react with certain wood extractives to
form a dark-blue, unsightly discoloration which becomes
sealed beneath the new finishing system.
Pentachlorophenol was commonly used in some
semitransparent penetrating stains and water-repellent
preservatives before it became a restricted-use
pesticide.