Michael Dresdner: Not all water stains will come out. Those that will often respond to oxalic acid. You can buy some oxalic acid crystals at the hardware store, make up a 6% solution (one tablespoon to one pint of warm water), and flood the piece with it — the whole piece, and not just the stains. Let it dry overnight, and repeat if necessary. The oxalic acid will remove stains and bring the oak back to its original color but will not bleach or discolor the oak. The acid will leave a dusty residue on the wood. Vacuum it off or sponge it off with lots of clean water before finishing. This process may raise the grain, so you may have to sand very lightly with 320 grit paper.

CAUTION: Oxalic acid is toxic and irritating in powdered form, so wear adust mask and goggles when handling it.

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