A: Here is what has happened. With the higher humidity of summer, and maybe the owner of the table using a bit more water than required to wipe the table top, there is a gain of moisture by the surface fibers. With this increase in moisture, the surface will try to swell, but the core, which has not increased in moisture, resists this swelling. This means that the surface fibers being restrained are under stress. So, the expanding or swelling surface fibers are forced to absorb the stress either by becoming crushed slightly or by ridging upwards at the joint. We often see this same effect with traditional wood strip floors.
Prevention is accomplished by avoiding rapid wetting of the surface. One way to do this is by using a better liquid water and water vapor proof coating; thin coatings are almost always poor. With a better coating any moisture will have time to move deeper 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 (probably impossible), that would help.
We can sand this ridge smooth now and refinish. But, if this is done when the moisture 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 the joint, or even a very small crack. So, check the moisture at the surface…it would be best to sand it smooth at about 7.0 percent MC, which is achieved at 38 percent RH.
If you do not sand it smooth now, the dry wintertime conditions will result in shrinkage (and maybe even a small crack at the joint), but the ridge will not totally go away. In other words, the ridge is not reversible by itself; you need to sand and refinish.
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