Wood bows or warps for many reasons including humidity and the natural stability of the wood. Some wood is more prone to warping and bowing than other types because of its' natural moisture content; the higher the moisture content, the more likely the wood is to warp. Typically, woodworkers stain and finish the top of tabletops and not the underside. Uneven finishing allows the underside to expand and contract normally while the top remains stationary and protected, which leads to bowing or warping.
Remove the table legs with a screwdriver or break the glue bond with a hammer and wood chisel.
Strip the finish off the table with a chemical wood stripper. Wash the table to remove the stripper residue.
Measure the length and width of the tabletop. Cut strips of 2-by 2-inch lumber 2-inches longer than the width of the table. Cut enough pieces so wood strips, known as battens, lay every 12-inches across the top and bottom of the length of the table.
Set up two sawhorses outdoors during hot weather or indoors where you can apply heat. Lay the battens across the sawhorses every 12-inches. Lay the tabletop across the sawhorses and battens with the bow facing up. The battens will extend 1 inch past the tabletop on each side.
Wet a thick towel and place it over the bow. Lay battens over the top of the wet towel and tabletop, lining them up with the battens under the tabletop. Make sure you place at least one batten so you span the center of the bow.
Position hand-screw clamps from the bottom batten to the batten directly over it and extend the end of the clamp along the batten toward the center of the table. Tighten the hand-screw clamps to apply firm pressure. Place another wet towel on top of the bow.
Set up a heat lamp aimed directly at the wet towels and bow or allow the table to sit in sunlight. Wet the towels and tighten the clamps each day for anywhere from three or four days to a week or more depending on the severity of the bow. Leave the towels, battens and heat source in place until the warp straightens.