Q: I made a table with a bunch of narrow pieces edge glued together. Now this summer the top is showing a ridge at every glue line—actually, it is hard to see, but I can feel it easily. Can you help?
A: Here is what has happened. With thehigher humidity of summer, and maybe the owner of the table using a bit morewater than required to wipe the table top, there is a gain of moisture by thesurface fibers. With this increase in moisture, the surface will try to swell,but the core, which has not increased in moisture, resists this swelling. Thismeans that the surface fibers being restrained are under stress. So, theexpanding or swelling surface fibers are forced to absorb the stress either bybecoming crushed slightly or by ridging upwards at the joint. We often see thissame effect with traditional wood stripfloors.
Preventionis accomplished by avoiding rapid wetting of the surface. One way to do this isby using a better liquid water and water vapor proof coating; thin coatings arealmost always poor. With a better coating any moisture will have time to movedeeper into the wood rather than creating a gradient right at the surface. Also,if we could reduce the amount of water used when wiping the table (probablyimpossible), that would help.
Wecan sand this ridge smooth now and refinish. But, if this is done when themoisture is a bit high, then when winter comes and the wood surface dries out,the surface fibers may (in a few cases) shrink, creating a depression at thejoint, or even a very small crack. So, check the moisture at the surface…it would be best tosand it smooth at about 7.0 percent MC, which is achieved at 38 percent RH.
Ifyou do not sand it smooth now, the dry wintertime conditions will result inshrinkage (and maybe even a small crack at the joint), but the ridge will nottotally go away. In other words, the ridge is not reversible by itself; youneed to sand and refinish.
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