Wood has a natural tendency to warp, especially in an unbraced piece like a tabletop or leaf. It occurs because of uneven expansion and shrinkage caused by moisture and heat. However, it is this same heat and moisture that can be put to work to reverse the curl and straighten the wood.
Remove all the old finish from the wood. Ask your hardware store for help in finding the right finish and stain remover.
Soak the wood. Once the finish has been removed, cover the bare wood with rags, sawdust or any other material that can be kept wet. Continue to soak it for four to five days.
Clamp the wood. After a thorough soaking, the wood should be pliable enough to clamp it and reverse the curl. Warped wood generally requires a clamp for about every 10 inches of length, so several clamps may be required.
Wait. Once the clamps are secure, place the wood in a warm, dry room for several days and wait.
Each day, loosen and then quickly re-tighten the bolts of the clamps. Doing so will prevent the formation of shrinkage cracks.
Refinish. Once the wood's curl has been reversed, refinish the surface as soon as possible to seal it against any moisture and prevent future warping.
The soaking time for the wood can be reduced by applying steam from a household iron through the dampened material several times each day. This will cause the wood to soak up more moisture, and cut your waiting time significantly.