by Professor Gene Wengert
Q.
Several days to a week after I glue up two pieces of wood with PVA adhesive, it appears that the glue line has oozed out--that is, there is a bump at the glue line, even though I sanded it smooth shortly after the glue had "set." Am I using the wrong adhesive?
A.
The glue itself becomes quite rigid after 24 hours in most cases. In fact, that is the cause of the problem. The wood pieces that you glued together must be a little too wet, either at the time of gluing or after you have sanded them. The wood then subsequently shrinks down a little bit--as a rule of thumb, 1% shrinkage for 2% MC change for oak (a very high shrinking wood) to 1% shrinkage for a 4% MC change for pine and other lower shrinking woods.
However, the glued area resists shrinking because the glue adds rigidity. The glue joint area stays the same size, even though the wood around it is shrinking. Hence, the bump that you see.
The cure is to wait a little longer before sanding and make sure that the wood MC is correct. Refer to the simple chart of RH vs. MC (below). Most houses and offices are at 30% RH in the fall, winter, and spring throughout most of the country.
RH MC
0% 0%
30% 6%
50% 9%
65% 12%
80% 16%
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.
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