(c) waterborne salts (such as copper quaternary ammonium
complexes, copper azole, and chromated copper arsenate
(CCA)) (Chap. 15). Note: Except for the all-wood
foundation, CCA-treated wood is not used in residential
construction.
Components for doors and windows are usually dip-treated
with a water-repellent preservative (WRP). The American
Wood Protection Association sets standards for pressure-
impregnated and dip-preservative treatment of wood
(AWPA 2008).
Wood treated with waterborne preservatives, such as cop-
per-based systems, can be painted or stained if the wood
is clean and dry. Bleed of preservative through finishes,
particularly latex-based paints and solid-color stains, can
occur if wood is still wet from the preservative treatment.
Allow wood to dry before painting; 1 week should be suffi-
cient. Wood treated with coal-tar creosote or other dark oily
preservatives is not paintable, except with specially formu-
lated finishes such as two-component epoxy paints; even if
the paint adheres to the treated wood, the dark oils tend to
discolor paint, especially light-colored paint. Wood treated
with a water-repellent preservative, by vacuum-pressure or
dipping, is paintable.
Fire-retardant- (FR-) treated wood is generally painted rath-
er than left unfinished because the FR treatment may darken
or discolor wood. FR treatment does not generally interfere
with adhesion of finishes; however, you should contact the
paint manufacturer, the FR manufacturer, and the treating
company to ensure that the products are compatible. Some
fire retardants may be hydroscopic and cause wood to have
high MC. FRs for wood used outdoors are formulated to
resist leaching.
Weathering
Weathering is the general term describing outdoor deg-
radation of materials and manifests itself physically and
chemically (for example, cracking and exfoliation of rock,
corrosion of metals, and photodegradation of organic ma-
terials). Ultraviolet (UV) radiation in sunlight catalyzes
photodegradation of organic materials exacerbated by mois-
ture, temperature change, freeze–thaw cycles, abrasion by
windblown particles, and growth of microorganisms. Degra-
dation occurs near the surface of wood, wood products, and
finishes.
Effect on Wood
Weathering takes many forms depending on the material;
wood and wood products initially show color change and
slight checking. Leaching of water-soluble extractives,
chemical changes, and discoloration of the surface by mi-
croorganisms cause color change. As weathering continues,
wood develops checks on lateral surfaces and checks and
cracks near the ends of boards, and wood fibers slowly
erode from the surface. Wood consists of three types of
organic components: carbohydrates (cellulose and hemicel-
luloses), lignin, and extractives. Weathering affects each of
these components differently, and physical and chemical
changes affect paintability.
Carbohydrates
Carbohydrates (cellulose and hemicelluloses) are polymers
of sugars and make up 55% to 65% of wood (Chap. 3).
Carbohydrates do not absorb UV radiation and are therefore
resistant to UV degradation. However, hemicelluloses and
amorphous cellulose readily absorb–desorb moisture; this
cyclic wetting and drying may cause different dimensional
change for EW/LW bands. Differential dimensional change
roughens wood, raises grain, and causes checks, cracks,
warping, and cupping. Fewer checks develop in woods with
moderate to low specific gravity than in those with high
specific gravity; vertical-grain boards develop fewer checks
than do flat-grain boards; and vertical-grain boards warp and
cup less than do flat-grain boards. To minimize cupping, the
width of a board should not exceed eight times its thickness.
The tendency to cup increases with the specific gravity and
width/thickness ratio.
Lignin
Approximately 20% to 30% of wood is composed of lignin,
a polymer that helps bond cellulose and hemicelluloses
within cell walls and bonds cells together. The volume be-
tween adjacent wood cells (middle lamella) is rich in lignin.
If exposed to UV radiation, lignin in the middle lamella, at
the surface of wood, begins to degrade within a few hours.
The changes are not obvious visually, but they affect the sur-
face chemistry of wood and thus adhesion of finishes. Lig-
nin photodegrades, leaving cellulose fibers loosely attached
to the surface. Further weathering causes fibers to be lost
from the surface (a process called erosion), but this process
is slow. Approximately 6 mm (1/4 in.) of wood is lost in a
century (Fig. 16–12). Erosion is slower for most hardwoods
and faster for low-density softwoods. Other factors such asChapter 16 Finishing of Wood
Figure 16–12. Artist’s rendition of weathering process
of round and square timbers. As cutaway shows, inte-
rior wood below surface is relatively unchanged.