Glue line bump

Q. We have a table top made of several edge-glued pieces of wood and after we finish it, we notice that the glue line appears to have a slight bump where it appears that the glue has squeezed out. Yet we sanded the top smooth prior to finishing, so we do not understand. I hope you can explain this.

A: I have studied this problem before and so I can tell you what is happening. When we edge glue pieces of wood together, there are spots where the joint is very tight and under pressure. The adhesive is hard enough that it does not squeeze out under this pressure, so after we sand the panel, it is indeed perfectly flat.
The pressure on the joint remains, however. Now, if we add heat to the panel, such as when curing a finish in an oven, then most commonly used adhesives will soften with this heat...the softening is at or near the surface, as the heat does not penetrate quickly to the core.

Now, the residual pressure in the joint will squeeze out a small amount of this heated, softened adhesive. Note that a water base finish will also swell the surface fibers, adding to the pressure. This affect will be seen the first few times the panel is heated. The cure is to reduce the heating, if possible, and to use an adhesive formulation that is less sensitive to heat.


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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.