UNITED STATES DEPARTMENT OF AGRICULTURE FOR
FOREST PRODUCTS (^L ABOruIOY ,-1F.TS DEPT T
MADISON 5. WISCONSIN.: .R E ISM-' July 1953
"" '' EPOSITORY1
COATINGS THAT PRtEN POSITODR
As wood dries, the surface fibers give up their moisture first and tend
to shrink but are restrained from so doing by the interior fibers, which
have not begun to shrink; thus drying stresses develop that tend to cause
checks. The tendency to check is more pronounced on the end-grain
than it is on the side-grain surfaces. A moisture-resistant end coating
is often used to protect such surfaces from checking during air season-
ing or kiln drying, especially with the more valuable items of wood that
are difficult to dry and hardwood logs that are to be stored for long per-
iods of time. End coatings should be applied as soon as possible to the
freshly cut end surfaces, for end checks, once started, tend to godeeper
into the wood as drying progresses. End coatings are also used to pre-
vent loss of moisture from the ends of short kiln samples, and for this
purpose the most highly water-resistant coatings are advisable.
To be satisfactory for large-scale use, an end coating should have the
1. Adequate resistance to water movement under all conditions
of temperature and humidity to which it may be subjected during the dry-
ing of the wood.
2. Ease of application under a wide range of temperatures.
3. Sufficient toughness and adhesiveness to withstand rough han-
dling and toprevent blistering and cracking during kiln drying.
4. Freedom from abrasive substances that may be injurious to
saws or cutters during machining of the wood.
5. Low cost to cover a given area.
The degree of water resistance required depends upon the species and
size of the wood. In general, greater water resistance is required for
the dense hardwoods and stock of large cross-sectional dimensions than
for the lighter hardwoods, the softwoods, and ordinary lumber. Highly
refractory woods, such as oak and black walnut, require the most highly
water-resistant coatings available.
End coatings may be divided into two classes: those that are liquid at
ordinary temperatures and can be applied without being heated, and
those that are solid at ordinary temperatures and must be applied hot.
Cold coatings can be applied readily to logs and lumber as well as to
kiln samples and dimension stock. Hot coatings are well suited for use
on small stock that can be easily handled, but they are not readily ap-
plied to logs and lumber.
Ordinary paints and varnishes are too thin for end coatings unless sev-
eral coats are applied. A number of special end coatings of the cold-
coating class are available from various manufacturers and dry kiln
companies. Many of these are heavily filled varnishes, but there are
other types such as heavy resin solutions, asphalt semimastic, ard
emulsified asphalt, wax, or synthetic resin. The properties of emulsi-
fied preparations vary widely, depending upon the amount of water and
type of emulsion. They are not satisfactory when subjected to rain or
freezing soon after application.
The effectiveness of commercial end coatings can be tested by compar-
ing them with either of the two end coatings originally developed by the
Forest Products Laboratory, an aluminized varnish and filled, hardened
gloss oil. Both have certain disadvantages for commercial use, but,
when properly made, htve a moderately high degree of water resistance
and will stand up under a variety of kiln conditions. Filled, hardened
gloss oil requires special preparation by a varnish manufacturer. Infor-
mation on suppliers or the formula for preparation will be supplied by
the Forest Products Laboratory upon request.
A wide varietyof aluminumpaints are available commercially and many
of them may be water-resistant enough to serve as end coatings. Abil-
ity to form a good adhering coating on wet wood is needed as well as
general water resistance. Even with the best coatings, two or more
coats are required to prevent endchecks. In workat theForest Products
Laboratory, a quick-drying phenolic resin tung-oil varnish was found to
be the best vehicle, and 1-3/4 to 2-1/4 pounds of aluminumpaste or pow-
der were used per gallon of varnish. Such coatings should not be made
up long before use, nor should they be overbrushed, or the desirable
"leafing" property of the aluminum particles will be decreased.
For small-scale use, heavy pastes such as roofing cement or white lead
in linseed oil can be used.
Cold coatings are usually applied by brush, although they can be sprayed
with proper equipment. They also can be applied to small items by hold-
ing the ends against a roller that is rotated while partly submerged in
the coating. Cold coatings should be allowed to dry a few hours before
being subjected to kiln temperatures.
Hot coating materials include pitch, asphalt, and paraffin. They are
low in cost and high in water resistance when applied in a single coat.
They maintain a high degree of water resistance even after prolonged
heating if the drying temperatures are kept about 30 F. or more below
the softening point of the coating. Standard softening points are deter-
mined by the ring-and-ball method (A. S. T. M. Designation: E28-51T).
Hot coatings can be prepared that will withstand any normal kiln tem-
perature, but those with higher softening points tend tobe less adhesive,
more brittle, and more difficult to apply than those with lower soften-
ing points. A coating with the 'lowest softening point that will safely
withstand thetemperatures encountered during the drying should be used.
Certain combinations of hot-coating materials can be mixed to obtain
desired softening-point characteristics and improved adhesiveness and
Coal-tar pitches are particularlyhigh in water resistance. The pitches
with higher softening points are especiallybrittle. For lowkiln temper-
atures, 155* F. coal-tar pitch is effective. One of the most water-resist-
ant coatings tested at the Laboratory is 213* F. pitch, but it is not read-
ily used alone because it is brittle when cold and does not adhere well
to the wood. Asphalt may be added to 213* F. pitch to increase the
toughness of the coating. One such combination, formulated at the For-
est Products Laboratory, consists of 60 parts (by weight) of 213* F.
coal-tar pitch, 25 parts of 155* F. coal-tar pitch, and 15 parts of 225*-
235' F. asphalt. This end coating is suitable for use with kiln temper-
atures up to 165* F.
Asphalt are tougher and more plastic than pitches having the same sof-
terrig point. Asphalts are derived from a number of sources, particu-
larlyfrom natural deposits and as abyproductof the distillation of petro-
leum. They vary in their softening point and water-resisting properties.
Many are suitable as hot coatings for kiln drying.
UNIVERSITY OF FLORIDA
3 1262 09216 7419
Paraffin has proved satisfactory as an end coating for stock being air
seasoned but cannot be used for stock being kiln dried because of its
low softening point.
Hot coatings can be easily applied to material of short length that can
be manipulated by hand. The temperature at which hot coatings should
be applied varies with the type used; pitch and asphalt coatings are ap-
plied at approximately 400 F. and paraffin at approximately 150" F.
The coatings can be applied by dipping the ends of the stock into the
molten coating to a depth of 1/2 inch, but better results are obtained by
firmly holding the ends against a power-driven roller that is rotated
while partly submerged in the heated coating. If the coatings are ap-
plied uniformly, a thickness of 1/20 to 1/16 inch is sufficient.
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