Q. We made some built-in furniture last spring with Douglas-fir. Now, about six months later, the wood is dripping sap on the carpet. We cleaned the sap off the wood surfaces, but now sap is oozing again. What happened and what can we do?
A. In most needle trees, including Douglas-fir, the wood is naturally full of resin, pitch or sap; these three words are synonyms. When drying the wood, in order to eliminate the sap bleeding in the final product, the kiln process will use a temperature of 160F or hotter for several days; this process is called “Setting the pitch.” This heat evaporates the portion of the sap that at room temperature will be soft and will move and drip; any sap still remaining in the wood will be hard at room temperature. (If the wood is exposed to hotter temperatures, like in direct sun or ceiling beam, then 180 F should be used to provide for better protection.)
(Note: If the pitch is not set, we will also see that the heat from sanding will soften the pitch and then the sandpaper will quickly clog or load.)
It seems clear that the wood you bought was not treated with the required high temperatures, and so your wood is totally unsatisfactory for the use you have. Perhaps your supplier can assume responsibility for correcting the problem.
At this point, there are two options: replacement or heat treating in place. With the heat option, you will have to heat the wood to 170F or hotter for a few days and also maintain 30 percent RH so the wood does not dry.
You may need professional help to get a heater this hot safely and avoid any fire risk. While heating, you will also need venting so that the evaporated pitch can be carried away from the heating enclosure. Overall, this is a very tough job.
Incidentally, I do not believe that any practical coating system exists for furniture that will seal the wood and keep the sap from bleeding out in this case.
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
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