I had multiple pieces warped badly, so I tried the water-in-the-sun trick. Basically, the plywood warps because one side is dryer than the other, causing it to shrink. Unfortunately, early April in western South Dakota still had chilly temperatures so I compensated by pouring boiling water into a bottle, spraying the bejesus out of the concave side, then dropping the pieces on smooth parts of workshop floor with my summer wheels on top to help straighten everything out. Long story short, it sort-of worked. More on that later.