Shellac (a natural product made from the secretion of
lac-producing insects such as Kerria lacca) and specially
formulated synthetic finishes block extractives bleed from
knots. Use shellac or synthetic knot sealers only over knots
and paint over them to protect them from water. Blocking
diffusion of extractives from knots is difficult, and no easy
fix is available other than the extra step of sealing knots
before priming. By doing this extra step, you can minimize
discoloration of white paint on knotty pine—but it is not
easy. If you want white, use knot-free wood. Difficulty seal-
ing knots is the main reason manufacturers cut out the knots
to make fingerjointed/edge-glued lumber.
Another option for knots is to use them to accentuate the
wood. Use a stain to bring out the color and make the knots
a part of the desired appearance.
Wood Products
Six types of wood products are commonly used on the exte-
rior of structures: (1) lumber, (2) plywood, (3) fingerjointed
wood, (4) reconstituted wood products (such as hardboard
and oriented strandboard (OSB)), (5) wood–plastic com-
posites, and (6) preservative- or fire-retardant-treated wood.
Each product has unique characteristics that affect applica-
tion and performance of finishes.
Lumber
Lumber (such as siding, trim, and decking) for exterior use
is available in many species and products, and several publi-
cations describe grades:
• “Standard Grading Rules for West Coast Lumber,” West
Coast Lumber Inspection Bureau, Portland, Oregon
• “Standard Grading Rules for Canadian Lumber,” Na-
tional Lumber Grades Authority, New Westminster,
British Columbia
• “Western Lumber Grading Rules,” Western Wood Prod-
ucts Association, Portland Oregon
• “Standard Grading Rules for Northeastern Lumber,”
Northeastern Lumber Manufacturers Association, Cum-
berland Maine
• “Standard Grading Rules,” Northern Softwood Lumber
Bureau, Cumberland Maine
• “Standard Specifications for Grades of California Red-
wood Lumber,” Redwood Inspection Service, Pleasant
Hill, California
• “Standard Grading Rules for Southern Pine Lumber,”
Southern Pine Inspection Bureau, Pensacola Florida
These publications are the basis for selecting wood to meet
codes. They give specifications for appearance grades (such
as siding and trim) and for structural lumber (such as fram-
ing and decking). Western redcedar and redwood are the
only species available in vertical-grain grades and saw-
textured surfaces (Table 16–1). Southern yellow pine and
Douglas-fir plywood are available in saw-textured surfaces.
Unless specified as vertical grain, the grade contains mostly
flat-grain lumber. Lumber used for board and batten, drop,
or shiplap siding is frequently flat grain. Bevel siding is
commonly produced in several grades. The highest grade of
redwood and western redcedar bevel siding is vertical grain
and all heartwood. Other grades of redwood and western
redcedar may be flat, vertical, or mixed grain and may not
be all heartwood. Grade is important because species, grain
orientation, and surface texture affect paint-holding charac-
teristics.
Descriptions of grades and pictures of many wood species
are contained in brochures published by trade associations
(such as Western Red Cedar Lumber Association, California
Redwood Association, Western Wood Products Associa-
tion, Southern Forest Products Association, and Northeast
Lumber Manufacturing Association) (Fig. 16–8), and these
brochures reference the grade rules. When specifying lum-
ber, refer to the grade rules for the product to ensure that the
product meets code requirements and use the association
brochures to get an idea of appearance.
Textures (roughness or smoothness) of wood surfaces af-
fect selection, application, and service life of finishes. Until
recently, a general rule of thumb for matching substrates to
finishes was to paint smooth wood and stain saw-textured
wood. This easy rule of thumb no longer applies. AlthoughGeneral Technical Report FPL–GTR– 190
General Technical Report FPL–GTR– 190Figure 16–8. Examples of trade association brochures
describing wood products.