paint. However, too many coats of latex paint can eventually
lead to adhesion failure of the primer.
In situations where catastrophic failure has occurred, refin-
ishing paint and solid-color stains may require extensive
surface preparation. First, scrape off all loose paint. In the
absence of lead-based paint, sand areas of exposed wood
with 50- to 80-grit sandpaper to remove the weathered sur-
face and to feather the abrupt paint edge. Wash the remain-
ing old paint using a commercial cleaner or a dilute house-
hold bleach and detergent solution to remove dirt and mil-
dew and rinse thoroughly (see Mildew). Prime the areas of
exposed wood, then top-coat. If the old paint has excessive
chalking, it may be necessary to re-prime (see Chalking).
Note: Do not sand lead-based paint. Use special precau-
tions if the old paint contains lead (see Lead-Based Paint).Table 16–4 summarizes the suitability and expected life of
commonly used exterior finishes on several wood species
and wood-based products. The information in these tables
gives general guidelines. Many factors affect paintability of
wood and service life of wood finishes. Table 16–5 summa-
rizes the properties, treatment, and maintenance of exterior
finishes.
Application of Finishes, Special
Uses
Porches, Decks, Deck Railings, and Fences
Porches get wet from windblown rain; therefore, apply a
WRP or primer to end grain of flooring, railings, posts, and
balustrade prior to or during construction. Primers and top-
coats for porch floors are formulated to resist abrasion.
Decks are usually finished with penetrating clears, lightly
pigmented clears, or semitransparent stains. These finishes
need more frequent application than does paint but do not
need extensive surface preparation, because they seldom fail
by cracking and peeling. Limit the application of semitrans-
parent stain to what the surface can absorb. The best appli-
cation method is by brush; roller and spray application may
put too much stain on horizontal surfaces. Unless specially
formulated for use on decks, solid-color stains should not be
used on decks or porches because they lack abrasion resis-
tance and they tend to fail by peeling.
Like decks, fences are fully exposed to the weather, and
some parts (such as posts) are in contact with the ground;
therefore, wood decay and termite attack are potential prob-
lems. Use lumber pressure-treated with preservatives or
naturally durable wood species for all posts and other fence
components that are in ground contact. When designing and
constructing fences and railings for decks and porches, ar-
chitects and contractors need to consider protecting exposed
end-grain of components to resist water absorption.
Film-forming finishes on fences and railings trap moisture
if the end grain is not sealed during construction. Figure
16–14 shows a railing 8 years after construction. Water
flowed down the railing and absorbed into the end grain,
and the paint kept the wood from drying. If railings are to
be painted, seal the end grain or use pressure-treated wood,
particularly where decay of wood is a safety hazard (railings
on decks and porches high off the ground).
Concerning the service life of naturally durable wood spe-
cies compared with wood pressure-treated with preserva-
tives, there are no absolute “rules.” However, for in-ground
contact uses and structural components of decks and porches
(beams, joist, and railings), pressure-treated wood is proba-
bly better and may be a code requirement in some areas. The
service lives of naturally durable and preservative-treated
woods are quite comparable in aboveground exposures, such
as decking boards. In selecting wood for porches, decks, and
fences, whether preservative treated or a naturally durable
species, consider the exposure conditions, design of the
structure, properties of the wood, and the finish to be used.
Wood weathering can be as much a factor in long-term
service life of decks and fences as decay. Protect naturally
durable wood species and preservative-treated wood with a
finish. Periodic treatment with a penetrating sealer, such as a
WRP or lightly pigmented deck finish will decrease check-
ing and splitting. Pigmented finishes retard weathering.Treated Wood
Copper-based preservatives (copper azole, ammoniacal cop-
per quat (ACQ), ammoniacal copper zinc arsenate (ACZA),
chromated copper arsenate (CCA)), creosote, and penta-
chlorophenol are common factory-applied preservatives.
Of these, wood treated with copper azole and ACQ is often
used to construct porches, decks, and fences. The treatment
has little effect on finishing once the wood has dried; species
and grain orientation affect finishing more than preservative
treatment does. Waterborne treatments containing copper
may maintain a brown color for approximately 2 years.
Some copper-based preservatives may have a water repel-
lent included in the treatment to give the treated wood better
resistance to weathering. Even if the manufacturer treated
the wood with water repellent, maintain it with a finish to
extend its service life. People often replace decking because
of weathering, not decay.
Creosote and pentachlorophenol are generally used for
industrial and commercial applications where applying a
finish is not considered practical. Creosote is oily, and wood
treated with creosote does not accept a finish. Pentachloro-
phenol is often formulated in heavy oil. Wood treated with
preservatives formulated in oil will not accept a finish.Marine Uses
The marine environment is particularly harsh on wood
because of wind-blown salt spray, abrasion by sand, and
direct and reflected UV radiation. Any of the types of finish
discussed previously can be used in marine environments.General Technical Report FPL–GTR– 190