Q: We are having a problem with raised glue joints in our solid wood (mahogany) panels and we would like to know what your recommendation is for the amount of moisture content that we could get by with, without causing this problem? This applies also to high-frequency gluing.
A: A raised glueline most commonly results because the wood between the gluelines is shrinking while the glueline itself is not. What causes the shrinkage in wood is that the wood is wetter than its environment, so the wood dries and this means that the wood will shrink. However, we might expect the glueline to shrink equally as much as the surrounding wood, if not a little more, as the water in the glue might (if we did not wait long enough between gluing and then sanding) mean more drying of the glueline and therefore more shrinkage. Indeed, this extra shrinkage does occur from time to time, giving us sunken joints. However, we have a special situation in your case in that the adhesive was cured quickly by the rf energy and heat and became quite rigid. As a result, the glueline is quite rigid and, at least in the short term, restricts the wood right at the glueline from shrinking or swelling when the moisture changes. So, when the sanded panel is exposed to a dry environment, the wood in the panel dries and shrinks while the glueline is unable to shrink as much due to the rigid glue itself. The end result is a raised joint.
The cure is to minimize any change in MC after sanding; stated another way, we need to equilibrate the panel to its final humidity conditions prior to sanding. Note that once the panel is finished with a moisture resistant finish, small changes in humidity will not cause large, rapid changes in moisture, so the joints should stay flat enough.
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