Wood Handbook, Wood as an Engineering Material

of synthetic polymers. Synthetic polymers do not penetrate
the cell wall, but form a thin film, seal the surface, and pro-
vide water repellency. Finely ground pigment gives color
and partially blocks UV radiation. Pigment, UV stabilizers,
and other additives give these finishes a service life of 2
to 3 years, but they lack sufficient pigment to inhibit UV
degradation of the wood. As with clear WRPs, they usually
contain a preservative to retard mildew growth.

Caution: Fungicides in WRPs and semitransparent stains

are toxic and may be herbicides; use caution to avoid skin

contact and breathing vapors, and protect plants and the

soil around them from accidental contamination.

Prior to changes in finish formulation because of VOC regu-
lation, paint companies formulated solvent-borne WRPs for
use as a pretreatment prior to priming. At this time, finding a
WRP formulated for this use is difficult. In fact, paint manu-
factures seldom honor a finish warranty, if customers apply
a WRP prior to using their finish, particularly if a different
paint company made the WRP. In spite of decades of re-
search showing the benefits of WRP treatment of wood prior
to priming, they are seldom used. Information on WRPs as
a wood treatment, prior to priming, is included in this chap-
ter in case a paint manufacturer markets a WRP specially
formulated for this use in the future. They are particularly
effective for improving the service life of paint on difficult-
to-paint wood species and decay-prone areas (Fig. 16–19).
Currently available WRPs can be used for sealing end grain

Protect wood and wood-based products from water and

sunlight prior to delivery and while stored at the construc-

tion site. Avoid contaminating them with dirt, oil, or other

contaminants. Finish wood as soon as possible after

installing it.

of lumber, edges of plywood, and back-priming and are dis-

cussed in several sections of this chapter.

Penetrating finishes that use paraffin oil as the solvent are

also available. These formulations penetrate wood, and the

oil helps improve water repellency. Paraffin oil is not a vola-

tile solvent; therefore, these finishes comply with air qual-

ity requirements. They are usually a good value, because

virtually all of what comes in the can ends up in the wood.

The service life is approximately 1 year, but they are easy to

apply. If an excessive amount is applied, the wood surface

may remain oily for a few weeks. Do not use them as a pre-

treatment prior to applying other finishes.

Application, New Construction

For new construction, the most effective method for apply-

ing a WR or WRP is to dip the entire board into the finish

(Table 16–5). If finish is roller or spray applied, back brush

following application to work the finish into the wood. Fin-

ish the back side of siding, particularly for highly colored

wood species (see Back-Priming).

When wood is finished following installation, apply lib-

eral amounts of WRP to all end grain areas, edges of panel

products, and other areas vulnerable to water, such as the

bottoms of doors and window frames. Coverage is approxi-

mately 6.1 m^2 L–1 (250 ft^2 gal–1) on a smooth surface or 3.7

m^2 L–1 (150 ft^2 gal–1) on a saw-textured surface. Smooth

wood will usually accept only a single coat; a second coat

will not penetrate the wood. WRP treatment lasts longer

on saw-textured surfaces than on smooth surfaces because

more finish penetrates the wood. As a natural finish, the life

expectancy of a WRP is only 1 to 2 years, depending upon

the wood and exposure. However, reapplication is easy,

particularly on decks and fences.

Refinishing

Clear and lightly colored finishes (penetrating natural fin-

ishes such as WRPs and lightly pigmented deck finishes) do

not peel; they fade, and if pigmented, the pigments erode.

As clear finishes weather, they lose their water repellency,

turn gray, and develop mildew. Lightly pigmented finishes

lose color. If not blackened by mildew, they can often be

prepared for refinishing by removing dirt with a stiff-bristle

brush. If discolored by mildew, wash the wood with com-

mercial mildew cleaner or dilute liquid household bleach

and detergent prior to refinishing (see Mildew).

General Technical Report FPL–GTR– 190

Figure 16–19. Effect of water-repellent preservative

treatment after 5 years of outdoor exposure. A, win-

dow sash and frame treated with a water-repellent pre-

servative and then painted; B, window sash and frame

not treated before painting.