Q: I have some lumber that is cupped quite a bit after it has been kiln dried. Is there a way to flatten this lumber by putting water on one face?

A: If you can flatten a cupped piece of lumber or a small cutting from this lumber, you are indeed quite lucky indeed. Indeed, flattening cupped lumber is almost impossible to do. The water techniques are not terribly effective.

The best chance at flattening can be achieved by quickly rewetting the convex side. Use hot water. It will try and expand, but it cannot do so. As a result, it will develop what is called compression set. (In brief, some of the cells that are trying to expand will squish.) Do not let water go beyond the surface (maybe 1/4 inch) on this convex side. In other words, do not wet for more than an hour at most. Remove the water and then let this convex side dry (but do not dry too fast, such as using a heater). As this side dries, it will shrink and hopefully the amount of shrinkage will be enough to result in a flat piece.

The problem with this technique is that it is hit and miss as far as how much water to add. Often, two treatments are no better than one. Further, flatness is not permanent; the piece will cup if rewetted.
An alternative method is to steam the piece, getting it "soaking wet" throughout as well as hot. Then bend it flat and then dry it while holding it flat. This again is not totally permanent and is hard to do.

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