Varnish Can Warp Wood

I’ve been finishing some shelf surfaces with Waterlox varnish. I’ve used it before with good results, but only on pieces I’ve finished assembling. In this instance, the shelves have yet to be installed into the final piece.

I only finished one side of each shelf, and after I put a 2nd coat on, I noticed one of the long edges of the wood was off the table. So I then eyeballed the shelf from the side and could see a clear warp in the shelf. The shelf is about 2 feet wide and 3 feet long, so it’s easy to spot a warp.

It initially perplexed me, because I typically associate warping with one side of the wood drying out faster than the other. In this case, that didn’t seem possible since, theoretically, the varnish should have stopped the exposed side from drying. If anything, I’d have expected it to warp in the other direction.

But then I recalled something Dad told me about some varnishes- that they actually shrink when they cure. Since the same thing happened on both shelves, this seemed like a more plausible explanation, despite the fact that I’d never seen this affect before. So I flipped the shelves over and applied the finish to the other side, hoping to straighten the shelf out.

The warp in the shelf, by the way, was a potentially major problem for me because the shelves aren’t installed yet. The way I’ll be installing them involves a spline running along the side that fits into a support. The support is dead straight. Matching a warped shelf to a straight support makes for a miserable experience.

Fortunately, I guessed right and applying the finish to the other side of the shelf has straightened the shelf out. So I won’t have to worry about fighting with the shelves during assembly. But I also learned something else to keep an eye on when it comes to wood finishing.