by Professor Gene Wengert
Q.
I have a question about warping. I made several humidors out of oak which I finished with a spar urethane and had great success, no warping. I was looking for a better finish so I tried tung oil which resulted in a warped top. I finished the inside with a urethane and the outside with the tung oil.
The top cupped towards the tung oil finish which was the lower humidity side. I sanded the top and evened out the humidity levels and the top straightened out. If I apply a urethane finish to the exterior or dry side now that it is straight, should it solve my warping problem?
A.
The warping you experienced is because the top side dried out faster than the bottom side. The urethane is a very good vapor barrier, so the humidity from the humidor is not a factor; the bottom side (or inside) didn't dry out very much at all. But the outside dried more quickly because the tung oil is not a vapor barrier.
The general advice is to finish both sides the same. Your suggested repair is okay.
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.
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