and are prone to embrittle over time. Latex paints (primers
and top-coats) permit water cleanup; oil-alkyd paints require
organic solvents for cleanup. Sufficient dry film thickness
on smooth-planed surfaces obscures wood grain and texture;
on saw-textured surfaces, some surface texture remains.
Application, New Construction
On smooth-planed wood, apply a primer and two top-coats
to achieve a 0.10–0.13-mm (4–5-mil) dry film thickness; on
saw-textured wood, primer and one top-coat may suffice.
As with solid-color stains, apply paints with brush, roller,
or sprayer. If using a roller or sprayer, back-brush to get an
even coating and ensure the finish wets the surface. Apply
the first coat of film-forming finishes (paint, latex semitrans-
parent stains, and solid-color stains) within 2 weeks after
installing smooth-planed exterior wood products; timely
application ensures good paint adhesion. Improve film adhe-
sion to smooth-planed flat-grain products, particularly those
species having abrupt grain transition, by wetting the wood
to raise the grain and scuff sanding (lightly sanding with
50–80 grit sandpaper) after it dries.
For woods with water-soluble extractives, such as redwood
and western redcedar, primers block extractives bleed into
the top-coat. Use a primer that is labeled to “block extrac-
tives bleed,” usually an oil-alkyd-based paint. Some manu-
facturers also formulate stain-blocking acrylic-latex primers.
Allow latex stain-blocking primer to dry for at least 24 to
48 h before applying the first top-coat. If the primer has not
fully coalesced, extractives may bleed into the top-coat. For
species, such as pine, that do not tend to have extractives
bleed, a quality primer is still necessary to give a good base
for top-coats. Follow the application rates recommended by
the manufacturer to achieve sufficient film thickness. A uni-
form primer coating having sufficient thickness distributes
wood swelling stresses and thus helps prevent premature
paint failure. Primer should cover approximately 6.1 to
7.4 m^2 L–1 (250 to 300 ft^2 gal–1) on smooth unfinished
wood; coverage is considerably less on saw-textured wood.
Apply two coats of acrylic latex paint over the primer. If ap-
plying two top-coats to the entire structure is not practical,
consider two top-coats for fully exposed areas on the south
and west sides and a single top-coat on other areas. Two top-
coats over a properly applied primer should last more than
10 years on smooth wood (Tables 16–4 and 16–5) and many
three-coat paint systems in test at FPL have lasted 20 years.
To avoid peeling between paint coats, paint manufacturers
recommend applying the first top-coat within 2 weeks after
the primer and the second top-coat within 2 weeks of the
first. If more than 2 weeks elapse between paint coats, it
may be necessary to wash the paint with mild detergent and
rinse thoroughly. If the primer has been exposed for several
months, it may need to be primed again prior to applying the
top-coats. However, some primer may not weather as quick-
ly and some top-coats may adhere well to weathered primer;
check with manufacturers for information on their products.
Avoid applying oil-alkyd paint to a hot surface in direct
sunlight and to a cool surface that the sun will heat within
a few hours. The heat causes the surface of the coating to
dry, trapping solvent in the film. The trapped solvent forms
a “temperature blister,” which usually occurs within a day or
two after painting. They do not contain water. Do not cool
the surface by spraying with water.
Apply latex-based waterborne paints when the temperature
is at least 10 °C (50 °F) and expected to remain above this
temperature for 24 h. (The dew point is a good estimate of
nighttime low temperature.) Most latex paints do not co-
alesce properly if the temperature drops below 10 °C
(50 °F). Oil-alkyd paint may be applied when the tempera-
ture is at least 4 °C (40 °F). Check with paint manufacturers
on the temperature requirements because some paints can be
applied at lower temperatures than these. As with oil-alkyd
paints, avoid painting hot surfaces in direct sunlight. Prior
to applying latex paints, the surface can be cooled with wa-
ter spray and allowed to dry.
Avoid painting late in the afternoon if heavy dew is expect-
ed during the night. Water absorption into partially cured
oil-alkyds or partially coalesced latexes can cause wrinkling,
fading, loss of gloss, and streaking.
Refinishing
In the absence of catastrophic failure such as cracking, flak-
ing, and peeling, solid-color stains and paints slowly erode.
A three-coat finish system (0.10–0.13 mm thick) may last
20 years on saw-textured wood. When the top-coats begin
to wear thin exposing the primer, reapply one or two new
top-coats. One coat may be adequate if the old paint surface
is in good condition. Surface preparation merely involves
washing the surface to remove mildew, dirt, and chalk.
Paint erodes at different rates, depending on the exposure
to sunlight; therefore, different sides of a structure do not
need to be painted on the same schedule. Paint on the north
side lasts twice as long as that on the south side (northern
hemisphere). When repainting, coverage should be approxi-
mately 9.8 m^2 L–1 (400 ft^2 gal–1).
Clean areas that are protected from sun and rain, such as
porches, soffits, and walls protected by overhangs. These ar-
eas tend to collect dirt that decreases adhesion of new paint.
Repainting protected areas every other time the structure is
painted usually gives adequate performance.
Do not paint too often. If paint is sound, but discolored
with mildew, wash it. It does not need repainting. Frequent
repainting may form an excessively thick film; thick oil-
based paint is likely to crack across the grain of the wood
(see Cross-Grain Cracking). Latex paints seldom develop
cross-grain cracking because they are more flexible than are
oil-based paints. Since latex paints have replaced oil-based
top-coats for most exterior applications, cross-grain crack-
ing is rare except for latex paint applied over thick oil-basedChapter 16 Finishing of Wood