TECHNICAL NOTE number 181 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY MADISON 5. WISCONSIN REVISED April I960 ZINO ( ^UNIV. OF FL UB ..DOCUMENTS t COATINGS FOR MINIMIZINQ CHANGES IN THE MOISTURE CONTENT OF WOOD ' CONT] - U.S. DEPOSITORY xaad_thc Shrinking and Swelling and the ^ C rnm pa ny-rrrp-i»»^»^«.-g. in wppj fha^ranfiP warping, checking, and weathering are brought about by changes in the moisture content. Such changes occur whenever wood is exposed to varying atmospheric conditions. Effective protection against fluctuating atmospheric conditions is furnished by coatings of various moisture re- tardant finishes, provided that the coating is applied to all surfaces of wood through which moisture might gain access. No coating is entirely moisture-proof, however, and there is as yet no way of keeping mois- ture out of wood that is exposed to dampness constantly or for prolonged periods. Tests of the moisture -excluding ability of coatings of many kinds have been made at the Forest Products Laboratory. (The method of test is described and more complete data are given in U. S. Department of Agriculture Circular No. 128 obtainable from the Superintendent of Docu- ments, Government Printing Office, Washington, D. C. , for 10^. ) Ta- bles 1 and 2 list coatings suitable for interior use only. The coatings in table 3 may be used for exterior as well as interior work; some are more durable than others. Ordinarily, moisture-excluding ability is only one of several properties that must be considered in selecting a coating for a given use. The data are for coatings only a few weeks old and not yet exposed to the weather. The effectiveness of many coatings improves slightly with age. Good exterior coatings either retain their maximum effectiveness for a considerable time or lose effectiveness slowly. As long as the original appearance and integrity of the coatings are retained, most of the effectiveness remains. Paint that is faded or chalking remains effec- tive if vigorous rubbing removes the chalk and discloses a glossy film underneath. Deep chalking, checking, or cracking indicates serious impairment of the effectiveness. The numerical values for percentage effectiveness are to be considered in a relative rather than an absolute sense because percentage effective- ness varies materially with the conditions under which exposure to mois- ture takes place. Coatings are much more effective, for example, against a brief exposure to water in liquid form than they are against one or two weeks of exposure to moisture -saturated air. The values for effective coatings (60 percent ot over) are reliable in the sense that they can be reproduced closely on repeating the test; values for ineffective coatings (less than 20 percent) must be regarded as rough approximations only. The percentages shown are based on average amounts of moisture ab- sorbed per unit surface area by newly coated and by uncoated wood pan- els subjected to a relative humidity of 95 to 100 percent for 14 days. In- terior coatings were tested on birch panels only, exterior coatings on white pine, redwood, Douglas -fir, and southern yellow pine. The data obtained lead to the following generalizations: 1. To retard the exchange of moisture between wood and air, a substan- tial coating that is relatively impervious to moisture is necessary. Merely "plugging the wood pores" is not sufficient. 2. The first coat (primer) applied to bare wood rarely forms a substan- tial, impervious coating. 3. Linseed oil alone is low in effectiveness even when a substantial coating has been achieved. The effectiveness of drying oils is greatly increased by incorporating resins with them (making varnishes), or by adding pigments (making paints). The more resin or pigment incorpor- ated, within practical limits, the greater the effectiveness. As a rule, paints are more effective than varnishes, and enamels (pigments added to varnish) more so than either. 4. Aluminum powder as a pigment makes especially effective and dura- ble coatings. It can be used with many liquids, but for exteriors the liquid itself should be suitable for exterior use. Aluminum paints may be used as complete coatings or as primers for coatings of other paints. 5. Asphalt and pitch paints are highly effective, inexpensive, and rea- sonably durable if well made, but they are dark in color and can rarely be painted over satisfactorily with light-colored paints. 6. Nitrocellulose wood lacquers are considered suitable for interior use only, and do not prove so effective as interior varnishes, though more effective than some exterior varnishes. Table 1. -- Moisture -excluding effectiveness of coatings suitable for in- terior use only Percentage effectiveness.*. Coating: Three coats of aluminum powder in gloss oil. ... 92 Heavy coating of paraffin. . . , 91 Three coats of rubbing varnish 89 Three coats of shellac 87 Three coats of enamel (cellulose lacquer vehicle) 76 Three coats of cellulose lacquer 73 Three coats of gloss oil bronzing liquid 12 Three coats of furniture wax 8 _ "Perfect protection would be represented by 100 percent effectiveness; the complete lack of protection in the case of uncoated wood, by zero. These data are to be considered only as approximations for relative comparison. Results vary significantly according to the kind of wood on which tests are made, and according to the thickness of the coat- ing. Table 2. -- Moisture -excluding effectiveness of some ty pical interior wall primers, flat paints, and semigloss wall p aintsj. Percentage effectiveness£ Coating: Wall primer A Wall primer B Wall primer C Two coats flat wall paint A Two coats flat wall paint B Two coats flat wall paint C Two coats semigloss wall paint A Two coats semigloss wall paint B Two coats semigloss wall paint C 1 coat 2 coats 57 79 41 69 23 35 .. 13 -- 4 -- 11 83 -- 52 -- 41 -3- Table 2. -- Moisture -excluding effectiveness of some typical interior wall primers, flat paints, and semigloss wall paint si. (Continued) Percentage effectiveness£ 1 coat 2 coats Coating: (Continued) One coat wall primer A + one coat flat wall paint A --67 One coat wall primer B + one coat flat wall paint B --44 One coat wall primer C + one coat flat wall paint C -- 27 One coat wall primer and semigloss wall paint A -- 82 One coat wall primer and semigloss wall paint B --69 One coat wall primer and semigloss wall paint C --27 ±The finishes designated A are made by one manufacturer, B by another, and C by a third. -Perfect protection would be represented by 100 percent effectiveness; the complete lack of protection in the case of uncoated wood, by zero. These data are to be considered only as approximations for relative comparison. Results vary significantly according to the kind of wood on which tests are made, and according to the thickness of the coating. Table 3. - -Moisture -excluding effectiveness of coatings suitable for ex terior or interior use Coating number Description- Percentage effectiveness£ 1 coat 2 coats 3 coats 1 2 Aluminum powder in asphalt or pitch paint vehicle Aluminum powder in No. 16 vehicle 39 88 78 92 98 95 3 Extra fine aluminum powder No. 16 vehicle in 94 •4- Table 3. --Moisture -excluding effectiveness of coatings suitable for ex- Coating number 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 terior or interior use (Continued) Description — Percentage effectiveness — 1 coat 2 coats 3 coats Aluminum powder in "alkyd" type synthetic vehicle 15 White lead in a vehicle similar to No. 16 62 One coat of No. 2 plus two coats of No. 17 39 White lead in No. 19 vehicle 24 Aluminum powder in No. 23 vehicle 9 Asphalt or pitch paint Aluminum powder in bodied linseed oil vehicle 26 One coat of No. 8 plus two coats of No. 17. 9 White lead in No. 23 vehicle 7 Aluminum in linseed oil 14 Aluminum and red lead in linseed oil 7 Linseed oil house paint con- taining zinc oxide and other white pigments with or without tinting colors 30 Phenol -aldehyde synthetic resin, 50-gal. varnish 5 Linseed oil house paint con- taining no zinc oxide, such as common "lead and oil" 20 Red lead in linseed oil 15 Ester gum resin, 3i-gal. spar varnish 6 Graphite in linseed oil 4 Red linseed oil barn paint, pig- ment 98 percent pure iron oxide 25 81 93 86 91 86 91 85 91 61 90 -- 90 84 65 89 62 83 57 77 75 69 73 49 73 57 70 56 67 37 65 58 64 53 56 -5 UNIVERSITY OF FLORIDA llllllilill 3 1262 09216 7252 Table 3. -- Moisture -excluding effectiveness oi coatings suitable for ex terior or interior use Coating number (Continued) Description— Percentage effectiveness — 1 coat 2 coats 3 coats 22 23 24 Red linseed oil barn paint, pig- ment Venetian red containing 40 percent iron oxide Ester gum resin, 75-gal. long- oil spar varnish Lin seed. oil containing paint drier 1 25 45 3 14 35 3 5 21 jjn this column, such entries as "No. 16 vehicle" or "Coating No. 17" re- fer to numbered coatings that are described elsewhere in the table. ^Perfect protection would be represented by 100 percent effectiveness; the complete lack of protection in the case of uncoated wood, by zero. These data are to be considered only as approximations for relative comparison. Results vary significantly according to the kind of wood on which tests are made, and according to the thickness of the coating. ZM 45264 F -6