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Q & A: Cupped Door Panels


Q & A: Cupped Door Panels


Recently I glued up some 1⁄4-in. panels for cabinet doors.My other life kept me out of the shop for a week and when I returned the panels had a 5⁄16-in. cup in them. I tried bending them back with clamps and that helped, but they still aren’t true. Is there anything else I can do?


Let’s start by trying to identify the cause of your problem. A panel that’s left lying on a workbench will have one side exposed to the air and the other flat against the benchtop. Any changes in relative humidity affect the exposed side sooner than the unexposed side. Increased humidity will cause a rise in the moisture content of the exposed face causing it to expand while the side against the bench remains dimensionally stable. This causes the panel to cup with the concave side down. Dry air would cup the panel in the opposite direction.

You might be able to reverse the cup in your panels by stickering them with weight on top and allowing the moisture content of the wood to equalize between the two faces. Sometimes spraying a little water on the concave side helps, too. The lesson here is to sticker your machined parts throughout the building process.

Panels made by resawing a board are prone to cupping due to interior tension created by the drying process. Like letting go of a stretched rubber band, the sawn face will shorten after resawing and create the cup. You can try to cure the condition by laying the cupped panel, concave side down, on a damp towel. Use a heat lamp as a heat source over the convex side. You want the panel to end up slightly cupped in the opposite direction. Be sure and stick around to monitor the process. When the panel cools and dries, it should be flat.

This story originally appeared in American Woodworker December 1999, issue #77.

December 1999, issue #77

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