Michigan
State University Extension
Home Maintenance And Repair - 01500571
06/24/03
Exterior Finishes for Wood
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 paintable. 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.
References
Materials here are from North Central Regional bulletin Wood Finishing
Selection and Application of Exterior Finishes for Wood, with Purdue
Extension and USDA Forest Service.
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