verse the weathering of
exterior wood, you can slow the
process dramatically by using the
right type of finish. Two basic
types of finishes or treatments are
used to protect wood surfaces
from weathering: those that form
a film or coating on the wood
(film-forming finishes) and those
that penetrate the wood surface.
Film-forming materials include
paints, solid-color stains, and
varnishes. Penetrating finishes
include preservatives, water
repellents, and pigmented
semitransparent stains.

Film-Forming Finishes

PaintLatex is Best
Paints form a thin layer over the
surface to which they are applied.
The pigments in the paint protect
the surface completely from the
damaging ultraviolet (UV) rays of
the sun and erosion. The finish
retards the movement of moisture
into and out of the wood and seals
in the natural resins. However,
paint will not prevent decay if
conditions are favorable for fun-
gal growth.

Oil-based or alkyd paints are es-
sentially suspensions of inorganic
pigments in an oil or modified oil
vehicle that binds the pigment
particles to the wood surface. La-
tex paints are suspensions of inor-
ganic pigments and various latex
resins in water. Acrylic latex res-
ins are flexible and durable. A
good acrylic latex outdoor house
paint will generally outlast a good
oil-based house paint. Latex
paints are very porous; they can
breathe while they shed water.
If water enters the wall from an
interior source, its more likely to
get trapped in the wood beneath
an oil-based finish, and it can
cause the paint to blister. Moreo-
ver, oil-based paints tend to be-
come
brittle.

Acrylic polymers are more resis-
tant to sunlight than oil-based
paints. However, woods like red-
wood and cedar have water-
soluble extractives that can bleed
through latex paints. The best way
to prevent this is to seal the wood
with an oil-based or stain-
blocking latex primer paint, then
topcoat with acrylic latex paint.

Wood is easier to repaint before
the first coat fails. Paints weather
away; self-cleaning paints
weather at a faster rate than other
paints. Ideally, wood should be
repainted when the topcoats have
weathered to the point that the
primer is just starting to show.
Painting too often can build up
too many layers of paint.

The north side of a building does
not need to be repainted as often
as other sides because it is ex-
posed to less direct sunlight. Ap-
ply one topcoat to this side instead
of two, or paint this side less of-
ten. The optimum thickness for a
primer and two topcoats is 4 to 5
mils
(1 mil = 1/1000 in.), about the
thickness of a sheet of newspaper.

If the paint has peeled, the easiest
way to prepare the wood for
repainting is by power washing.
Power washing or sanding (with
5080 grit sandpaper) also
improves the ability of new
smooth siding to hold paint and
removes weathered fibers of
unfinished wood that have been
exposed to sunlight for more than
a few weeks.

Before applying paint, treat wood
siding and trim with a paintable
water-repellent preservative. This

Life Span of Various Finishes
______________________________
Water repellents 612 months
Clear water-repellent
preservatives 1-2 years
Pigmented water-repellent
preservatives 2-3 years
Varnish 23 years
Solid-color stains 37 years
Semitransparent stains 38 years
Paints 710 years
May 2000 ote: The life span of water repellents,
water-repellent preservatives, and
semitransparent stains can be extended
by refinishing if discoloration caused
by mildew or other factors is a
problem.

can be done by brush after the
siding or trim is in place or by
dipping the wood before
installation. All lap and butt joints
of solid wood and all panel edges
should be well saturated.

Allow the treatment to dry for at
least 2 warm, sunny days before
applying the primer. If the wood
has been dip treated, let it dry for
about a week. Two coats of a good
quality acrylic latex house paint
should be applied over the primer.
In general, paint quality is directly
related to price.

Brush application is always super-
ior to roller or spray application,
especially with the first coat. Each
coat should be applied within 2
weeks of the previous one, to avoid
formation of a slick soap-like or
chalky substance on the surface. If
more than 2 weeks have elapsed,
scrub the paint with water and a
stiff-bristled brush before applying
the next coat.

Temperature is important. Do not
paint on a cool surface that will be
heated by the sun within a few
hours or at the end of a cool day
when heavy dew will form at night.
The temperature should stay >40�F
(>4�C) for 24 hours for oil paints
and >50�F (>10�C) for latex.

Solid-Color Stain
Solid-color stains also form a film.
They look and act like thin paints.
Use a primer and two coats of
stain. Refinishing may require only
one coat. Like paints, latex solid-
color stains are usually more
durable than oil-based stains.
Solid-color stains are preferable to
paints when the wood is refinished
frequently.

Varnish
Because varnishes are clear, they
require frequent maintenance to be
attractive. Sunlight degrades both
the varnish and the wood fibers
directly beneath it. Some varnishes
have chemicals that partially block
UV light, but all varnishes will
eventually crack, peel, and flake,
taking along the fibers of photo-
chemically degraded wood. The
wood needs to be cleaned and
revarnished as soon as this break-
down occurs. Varnish lasts only 1
to 2 years on wood exposed to full
sun. The life span of varnish can be
extended by first staining the wood,
then applying many coats of
varnish.

Penetrating Finishes
Penetrating finishes absorb into
the wood, saturating the surface
fibers and partially or completely
filling the surface pores. Many
penetrating finishes contain water
repellents, usually in the form of
paraffin wax dissolved in mineral
spirits.

Water repellents are usually clear.
They can be used alone as a natural
penetrating finish or as a treatment
prior to painting. Check the label
of the finish to make sure that it is
paintable.

Many clear, penetrating finishes
contain wood preservatives in
addition to water repellents; these
penetrating finishes are much
better for outdoor use. The
preservatives control the growth of
mildew and other fungi. Some
preservatives also discourage insect
infestation. This is especially
important in moist, shady
locations.

Semitransparent Stain
Oil is Best
Semitransparent stains are pene-
trating finishes with inorganic
pigments. High quality stains
contain wood preservatives and
water repellents. Semitransparent
stains penetrate the wood without
radiation that would otherwise
degrade the wood. The properties
of the pigment, resin, preservative,
and water repellent determine the
durability of exterior stains.

Penetrating stains perform best on
weathered or coarse-textured wood.
If you are finishing smooth wood,
powerwash, sand, or wet the
surface to relieve stresses and open
the surface pores. Unlike paints,
stains can be applied to weathered
surfaces without preparing the
surface. If the surface is dirty or
has mildew, clean it with bleach
and detergent before applying the
finish. Penetrating stains are
suitable for siding, trim, decking,
fences, and roof shingles.

Stains may be applied by brush,
spray, or roller. To prevent lap
marks, finish the board or panel in
one working session.

Working in the shade will give the
best results because longer drying
time means greater penetration of
the stain.

For best results, rough sawn or
weathered lumber should be treated
with two coats of penetrating stain.
Apply the second coat before the
first is dry. If the first coat has
completely dried, it may seal the
wood surface so that the second
coat scarcely penetrates. To prevent
formation of a film, an hour after
applying the second coat, use a
cloth, sponge, or dry brush lightly
wetted with stain to wipe off excess
stain that did not penetrate the
wood. Remember that sponges or cloths soaked with oil-or alkyd-
based stains are particularly
susceptible to spontaneous
combustion. They should be
immersed in water or sealed in an
airtight metal container.

Natural Finish
To some, a natural look means
rough, gray, and weathered. To
others, a successful natural exterior
wood finish is one that retains the
original, attractive appearance of
the wood with the least change in
color and the least masking of the
grain and surface texture. In this
case, the finish should inhibit the
growth of mildew, protect against
moisture and sunlight, and not
change the surface appearance or
color of the wood.

The weathered look can be
obtained in several ways. Do
nothing until the wood turns gray,