Here's an easy way to correct warps. Wipe on a light treatment of Wood Elixir on the 'cupped' or concave side of the wood. Allow 24 hours to cure. On a day that is anticipated to be warm and sunny, get up early and take the table top outside. Place it concave side down (hump side up) on a patch of grass where the dew has not yet evaporated. When the sun hits it, it will start to flatten out. When it is flat, seal the unfinished underside of the table with a coat of shellac.