I saw this interesting artile


The woodworker/boatbuilder mythological belief that linseed oil waterproofs wood was proven false close to 100 years ago. The U. S. Department of Agriculture Circular No. 128, October, 1930, Effectiveness of Moisture-Excluding Coatings on Wood, described work done at Forest Products Laboratory beginning in 1914. Their conclusion then, and in all their subsequent work, was that while some treatments slowed the dimensional changes of wood as it absorbed water “...a linseed oil coating had little effect.” During World War I (1914-1918) much trouble was encountered with wood airplane propellers getting out of balance and changing shape as a result of moisture changes. FPL developed covering propellors with aluminum leaf (extremely thin aluminum foil) that performed best at preventing shape changes and was used. The 1930 Bulletin described work after World War I that included coatings of five coats of linseed oil plus two coatings of floor wax and, also, soaking wood in linseed oil. They also tested the complete range of paints, enamels, and coatings available at the time. They had their best results with aluminum paints and some varnishes. Again they concluded, “Coatings or treatments with linseed oil, floor wax, and the like were low in effectiveness.” FPL research on the subject continues today. The latest publication I have is Research Paper 462, December, 1985. Results reported include treating wood with linseed oil and linseed oil dissolved in mineral spirits. In both cases, the treated wood absorbed more water in a high humidity (90%) chamber than did untreated wood A modern myth is that epoxy resin penetrates sound wood and that epoxy coating prevents water from being absorbed by wood. Except for end grain, and especially plywood edge grain, eopoxy does not penetrate sound wood to a significant depth. This is true for epoxy thinned with large amounts of solvent, except in contrived experiments with balsa wood, a natural sponge. FPL found that three coats of epoxy resin gave the lowest water absorption of any of the coatings they tested, but after 60 days at 90%RH the epoxy-coated wood had absorbed 73% as much moisture as if it were uncoated. Boats are immersed in water, so I measured water absorption while immersed of ¾” pine coated with three heavy coats of epoxy resin. These coatings were about twice as thick as used at FPL. In fact, their total weight was a little over 30% of the weight of the uncoated wood. Immersed in water they absorbed 20% by weight of water in 200 days. That is the water content required to sustain rot organisms. There are two other requirements for rot to occur-presence of the organisms and oxygen to sustain them. The rot spores are astronomically larger than water molecules, so if the wood did not have them to begin with, epoxy coating would likely keep them out. As to oxygen, the oxygen molecules are far larger than water molecules and do not have the unique properties of water molecules that make them capable of penetrating any organic material-natural or synthetic. I learned this the hard way while supervising spending a couple of million dollars around 1960 on a DuPont research project to make building products and pipes of resins reinforced with kraft paper. Dry kraft paper is a pretty strong, stiff material; wet, it is like the proverbial dishrag. The chemists doing the experiments tried every polymerizable mix they could conceive, including epoxy resins, to make products that wouldn’t lose their stiffness in long-term contact with water. The research director held his head in his hands when I reported that we had had zero success. I also coated exterior fir plywood (three equal thickness plies) and underlayment lauan plywood (thin skins and thick core) with about 30% of their weights of epoxy resin. The fir plywood took about 1½ years of soaking to reach 20% water absorptiion, but the lauan hadn’t reached that point in two years and looked as if it never might. This resuly hints that the glue lines between skins and cores of plywood are quite effective barriers to water absorption. David W. Carnell 11 March 2004