Q & A: Cupped Door Panels

Q & A: Cupped Door Panels

Q:

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?

A:

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.