Raised Glue Joints after Staining

Why would finishes cause glue joints to swell and telegraph? And what's the fix? April 24, 2012

Question
I recently had 50 maple plank table tops made for me. I order these bald and then I put the finish on. The tops were delivered about two weeks ago. I wrapped them in six mil plastic. When I checked the moisture before finishing, they were at 6-7%. The glue used on these is a UV cured glue. The company that made these told me they recently tweaked the glue to make it more flexible.

Before finishing I noticed some raised glue joints and sanded them smooth. I also noticed some glue joints that looked like an earthquake took place and filled them. Then I noticed that when I put my dye stain on, cut with acetone, it raised the glue joints again. I tried cutting with a weaker solvent, xylene based - same result. Then I tried denatured alcohol to cut it - same result.

Anyone know what is causing this? Anyone know what the cure is? I think my only option is to build them up with polyester and then sand smooth and put my topcoat on.

Forum Responses
(Adhesives Forum)
From contributor A:
The glue is flexible. As the wood expands due to moisture, it crushes the glue joint, forcing the soft glue out of the joint. This happens with typical white woodworking glues. It's usually a fool's errand to keep cutting it back, thinking you will eventually sand away the problem. Your idea of burying it under a heavy coat of polyester sounds like a reasonable solution. Some people refer to this as glue creep (Gene doesn't approve of this word misuse), or print through/telegraphing when it keeps appearing in the topcoats.



From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
The UV cured adhesives are usually more stable and would not move or allow the wood to move, even with a solvent. But I am not sure how such an adhesive would work when fastening planks. How does the UV penetrate the joint to reach the adhesive in the interior to get it to cure?

So, what is causing the surface to swell, creating pressure which results in the raised joint? Or causing the core to shrink and getting the same effect? Why not try a panel that is wetted briefly with a damp rag? That will swell the surface, but not affect the adhesive.

Because alcohol is NGR and would not cause swelling of the wood, perhaps the environment is too humid and this is what is causing the swelling. Try some panels that you handle the same way as a stained panel, but do not dye or stain itů Just expose it to the same conditions. Finally, take a chisel and drive it along a joint lengthwise. Does the glue joint fail or is there wood failure? Is the wood failure only at the surface?

All in all, it sounds like wood movement issues, with the glue being flexible enough (or perhaps not a strong enough joint) to offer no resistance to wood movement.