Caution: Oxalic acid is toxic; take care when using it.
(It is the poison in rhubarb leaves.)Knots
Knots in many species contain an abundance of resins and
other highly colored compounds. These compounds can
sometimes cause paint to peel or turn brown. Eliminating
paint discoloration caused by extractives in knots is difficult
because some of the extractives are soluble in oil-based
primers and diffuse through them. Latex-based formula-
tions do not block them either. Coat the knot with shellac or
specially formulated knot sealer. Do not use varnish to seal
knots; varnish is not formulated for this use. After sealing
knots, apply primer and two top-coats. Knots usually check
as wood dries; if the checks form after the wood has been
painted, the checks cause the paint to crack (see Wood
Extractives).Pitch
Pitch and other resins are one of the defense mechanisms
that a tree uses to protect itself from harmful pathogens and
insects following injury. When a tree’s bark is damaged,
pitch flows into these areas to protect the wound
(Fig. 16–29). Pitch exists as a normal part of the wood of
pines (Pinus spp.), spruces (Picea spp.), larches (Larix
spp.), and Douglas-firs (Pseudotsuga spp.), and it can be
found in specialized wound structures called pitch pockets
in the wood of most softwood species. Pitch is a solution of
natural rosins and turpentine in species such as spruce, pine,
and fir. It remains in the lumber from these species. The ease
with which it exudes to the surface of lumber depends on
the amount of turpentine in which the pitch is dissolved and
the temperature (that is, the more turpentine, the more fluid
the pitch; the higher the temperature, the more fluid
the pitch). Pitch exudation can occur in isolated spots
(Fig. 16–30a) or in large pockets or seams (Fig. 16–30b).
When pitch bleed occurs, high temperature is the cause. If
the temperature at the surface of the wood increases, usually
from being exposed to direct sunlight, the pitch oozes to the
surface. If the wood is finished, the pitch may exude through
the coating or cause the finish to discolor or blister.
The only way to prevent pitch bleed is to remove the turpen-
tine from the wood during lumber processing. Depending on
the species, specific kiln schedules can be used to drive off
most of the turpentine, thus “fixing” or “setting” the pitch
(making it less fluid). However, not all end uses of lumber
require pitch to be set; construction grades of lumber, even
if kiln-dried, seldom have the pitch set. This is usually not a
problem for construction grades because the wood surface is
seldom visible. The difficulty occurs with appearance grades
of lumber, such as for siding and trim.
Kiln schedules for setting pitch involve higher temperatures
and last longer than normal drying schedules. For a com-
plete guide to drying schedules, refer to publications such
as the Dry Kiln Operator’s Manual.
Pitch can be removed in several ways, depending on how
fluid it is. If the pitch has not hardened (it still contains a lot
of turpentine), remove it with turpentine or mineral spirits.
Once it has hardened, scrape it off with a putty knife or
paint scraper. However, if the pitch is still soft, such pro-
cedures smear it over the surface of wood or paint. Let it
harden, and then scrape it off. After removing pitch, sand to
bare wood, spot prime, and top-coat. Shellac seals extrac-
tives but not pitch. Paint will not prevent future bleeding of
pitch during periods of high temperature. If pitch is a recur-
ring problem, it may be necessary to replace the board. One
should note that many paints, particularly oil-alkyds, fade as
they age and repainting the spots where pitch was removed
may show color differences.Chapter 16 Finishing of Wood
Figure 16–28. Iron stain on newly installed wood sid-
ing. Poor quality galvanized nails corrode easily and,
like uncoated steel nails, usually cause unsightly stain-
ing of the wood.
Figure 16–29. Pitch flow from wound.