Wood Finishing
Textured plywood surfaces are probably the most
common for exterior siding. Sanded and roughsawn plywood
will develop surface checks, especially when exposed to
moisture and sunlight. These checks, coupled with the
flat grain pattern (wide bands of dark, dense latewood)
characteristic of nearly all plywood, can lead to early
paint failure. These paint failures can be minimized by
the use of top quality acrylic latex paint systems.
Semitransparent Stains
Unlike paints, semitransparent penetrating oil-base
stains cannot check and peel from plywood surfaces. These
stains penetrate the wood and do not form a continuous
film or coating like paint. Semitransparent penetrating
stains allow most of the wood grain to show through, and
the color can be controlled by pigments added to the
stain. Penetrating stains also perform well on weathered
surfaces. New, smooth surfaces may also be stained.
Oil-base penetrating stains have a longer life expectancy
when properly applied to roughsawn or weathered surfaces.
Semitransparent stains may be brushed or rolled on.
Brushing should give better penetration and performance
especially on textured surfaces. These stains are
generally thin and runny, so application can be a little
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. Working in
the shade is desirable because the drying rate is slower.
The penetrating stain should be stirred frequently during
application. One gallon will usually cover about 300-400
square feet of smooth surface and from 150-250 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 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 dries
completely, it may 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
may form an unsightly surface film and glossy spots.
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. If only one coat of penetrating stain is
used on new wood, its expected life is 2-4 years, but
succeeding coats will last longer.
Refinishing semitransparent penetrating oil-base
stains is relatively easy. 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 surface should be
free of mildew. A longer service life can be expected for
penetrating stains the second time they are applied since
they will penetrate the many small surface checks which
open up as wood weathers.
Water-repellent preservatives can also be used as a
natural finish for plywood. Water-repellent preservatives
are mixtures of a solvent such as mineral spirits or
other paint thinners, wax, a resin or drying oil and a
wood preservative. These finishes, like semitransparent
stains, penetrate the wood and do not form a surface
film, so peeling will not be a problem. Since they do not
contain any coloring pigments, they will allow he natural
wood color and grain to show through. Expected service
life is only 1 to 2 years, and frequent reapplication is
necessary to protect the wood surface.
Water-repellent preservatives are best applied by
dipping, but brush treatment to the point of refusal is
also satisfactory. It is especially important to apply
liberal amounts of the solution to all joints or other
potential places where moisture might accumulate. Be
certain to treat the horizontal bottom edges of any
panels.
Refinishing water-repellent preservatives is
accomplished by simply cleaning the old surface with a
bristle brush and applying 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. 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.
Water repellents are sometimes used in the same manner
as water-repellent preservatives. However, they do not
contain a wood preservative and will not protect against
surface mold and mildew.
Note.: Steel wool and wire brushes should not be used
to clean surfaces which will be finished with
semitransparent stains or water-repellent preservatives.
Small iron deposits left on the surface can react with
certain wood extractives to form a dark-blue, unsightly
discoloration which is sealed beneath the new finishing
system.
Paints
In some cases, painting of plywood is required or
desirable. Top quality acrylic latex paints are the best
choice for exterior surfaces. For overlaid or MDO
plywood, remove all loose paint with a stiff bristle
brush and then scrub with a soft brush or sponge and
water. Rub your hand against the cleaned surface to
determine if any residues remain. When necessary,
scrubbing with a detergent or paint cleaner will usually
remove additional residues. Then rinse well and allow to
dry before repainting.
If non-overlaid plywood is to be painted, follow
these tips. First, brush a liberal quantity of water-
repellent preservative or water repellent onto all the
edges of the plywood sheets. The surface should also be
treated in the same manner. The water repellent will help
reduce wood's tendency to absorb moisture through the end
grain and surface lathe checks. Allow the water-repellent
preservative or water repellent to dry for at least two
warm days. Then prime the plywood surface with a high
quality paint recommended for use on woods which contain
extractives. The primer should be applied thick enough to
obscure the wood grain pattern. Two coats of a high
quality acrylic latex house paint should be applied over
the primer. Allow at least two days but no longer than
two weeks between the primer and top coat. The primer and
top coat should be compatible and preferably from the
same manufacturer. Always remove the mildew before
refinishing.
Refinishing painted plywood requires proper surface
preparation if the new paint 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 the remaining paint
with a brush or sponge and water. Household bleach (50/0
sodium hypochlorite) used at the rate of 1 cup of bleach
to 3 cups of water will remove mildew. Rinse the surface
with clean water. Wipe the surface with your hand. If the
surface is still dirty or chalky, scrub it again using a
detergent or paint cleaner. Rinse the surface thoroughly
with clean 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. One or
preferably two top coats should follow.
Reconstituted wood products are those made by forming
large sheets, usually 4 by 8 feet, from small pieces of
wood or pulp. Reconstituted wood products may be smooth
or textured to look like standard lumber. Depending upon
the basic wood component used in their manufacture,
reconstituted wood products may be classified as
fiberboard or particleboard.
Reconstituted wood products can be purchased
unfinished, primed, with a top coat, or stained. Only
some fiberboards and particleboards are manufactured for
exterior use. Be sure to check with the supplier, and
follow the manufacturer's directions in using and
finishing these board type products. Film-forming
finishes such as acrylic latex paints and solid color
acrylic latex stains will give the most protection. Other
finishes such as semi-transparent stains allow a more
rapid deterioration of the surface to take place.
To paint hardboard and particleboard, follow good
finishing practices as recommended for plywood. Be sure
to seal all edges with a water-repellent preservative or
water repellent. The surface should be treated with the
same solution. Then apply a primer coat recommended for
use over wood, followed by at least one high quality top
coat of acrylic latex house paint.
Some reconstituted wood products may be factory
primed with paint and some may even have a top coat.
Factory primed boards should not be allowed to weather
for more than a couple of weeks before top coating. If
excessive weathering does occur, clean the factory primed
surface, and then reprime and follow with two top coats.
Reconstituted wood products may be refinished by
following those procedures recommended for plywood.
Care must be exercised in preparing a wood floor since
finishing will accentuate any defects, irregularities or
roughness. Even irregularities that can scarcely be seen
before finishing become conspicuous afterward. Nothing
can be done later which will make up for the defects of a
poor sanding job. Unless prefinished, flooring usually
has not been sanded by the manufacturer and bears across-
the-grain ridges left by planing. These marks will mar
the appearance when the finish is applied. Moreover, if
much time passes between the final sanding and the
application of finish, some roughness may develop from
raising of the wood grain because of changing moisture
content. Floors should be sanded shortly before finishing
is begun.