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 TECHNICAL NOTE number 186 
 
 UNITED STATES DEPARTMENT OF AGRICULTURE F OREST SFRV ir F 
 
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 FOREST PRODUCTS LABORATORY <lTS OEPT 
 
 MADISON 5. WISCONSIN REVISED July 1953J 
 
 
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 COATINGS THAT PREVENT END C 
 
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 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 go deeper 
 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 
 following qualities: 
 
 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 to prevent 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 
 thedense 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. 
 
 Cold Coatings 
 
 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, and 
 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, have 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 variety of aluminum paints 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 end checks. In work at the Forest 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 aluminum paste 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. 
 
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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 Coatings 
 
 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: E28-51 T). 
 
 Hot coatings can be prepared that will withstand any normal kiln tem- 
 perature, but those with higher softening points tend to be 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 the temperatures 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 
 toughness. 
 
 Coal-tar pitches are particularly high in water resistance. The pitches 
 with higher softening points are especially brittle. For low kiln 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. 
 
 Asphalts are tougher and more plastic than pitches having the same sof- 
 tening 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. 
 
UNIV 
 
 ERSITY 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. 
 
 2 M 49724 F Agriculture-Madison 
 
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