Lift Dark Stains Out of Finished Wood With an Oxalic-Acid Paste

The Problem

A dark stain shows up in a wood tabletop — a chemical mark, the gray-black ghost of water or something that reacted with the wood — and the standard fix is grim: sand through the finish, bleach or re-stain, refinish, and hope the repair blends. For one small stain on an otherwise good surface, that's a whole weekend against a blemish.

The Technique

Oxalic acid is the classic wood-bleaching agent for exactly these dark marks, and it's the active ingredient in a cleanser many kitchens already have (Bar Keepers Friend is the common one).

1. Mix the powdered cleanser with a little water into a thick slurry.
2. Spoon it onto the stain — only the stain — and leave it there for about an hour.
3. Wipe it off with a paper towel carrying plenty of water, and a light touch. The product is an abrasive cleanser: dragged across the surface dry, it's sandpaper on your finish.
4. Judge nothing yet. The result looks unimpressive wet; the stain keeps fading as the wood dries, and over the following couple of days it can fade to invisible.
5. Repeat if a shadow remains.