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If the frame is made from a softwood, such as pine, you may be able to straighten it by bracing it from the back. Connect each pair of diagonal corners with a wood or metal brace, screwing the brace to the corners with wood screws to draw the frame into alignment.
When you wet the wood, the glue holding the corners may fail, and you may have to re-glue them after straightening the wood.
Warping is a problem that can affect anything made of wood. It occurs because the fibers in a typical piece of wood come from different parts of the tree, and some dry out more quickly than others. The fibers shrink as they lose moisture, creating tension that pulls the board into a curve. To un-warp a board -- which isn't always possible -- you need to replace some of the lost moisture to restore flexibility. You should then be able to bend the wood back into shape and hold it steady with clamps while it dries out.
Remove the picture and the glass from the frame. Lay the frame on its face on a flat piece of 3/4-inch plywood that is slightly larger than the frame. To avoid damage to the finish, put a towel on the plywood before laying down the frame.
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Cut an absorbent rag into long strips, using scissors. Soak the strips in water, wring them out and lay them on the back of the frame, completely covering the wood.
Arrange 8 C-clamps evenly around the perimeter of the frame and clamp the frame to the plywood. Tighten the clamps enough to make the wood begin to straighten, but not enough to draw it against the plywood.
Moisten the rags daily with a spray bottle of water, and each day, tighten all the clamps about a quarter turn. The idea is to draw the frame gradually flush with the plywood without breaking or cracking the wood.
Stop moistening the rags when the frame is flush against the plywood, which may take a week or more. Keep the frame clamped for a week to let the wood dry out. If all goes well, and the warp wasn't too severe to begin with, the frame should be straight when you remove the clamps.