There are two types of watermarks on furniture: white and black. White marks are in the finish and can usually be removed by rubbing with an abrasive. Black marks are stains in the wood and can usually be removed with oxalic acid, but only after the finish has been removed first.
Oxalic acid is a type of bleach. It’s not the same as household bleach or two-part bleach (sodium hydroxide and hydrogen peroxide), which is sold in separate containers. Household bleach will remove dye stain but not black watermarks. Two-part bleach will remove black water marks but also all the natural color from the wood, which you rarely want to do.
The accompanying before-and-after photos were taken by Bob Stevenson. They show an extremely bad case, together with the power of oxalic acid.
Here’s how to remove black watermarks
- Strip the finish.
- Dissolve oxalic-acid crystals to a saturated solution in hot water. (Saturated is when no more will dissolve.)
- Brush the solution onto the entire surface of the wood, not just over the marks. (This ensures that the color will come out even.)
- Let the solution dry back to crystal form.
- Thoroughly wash off the crystals. (Don’t brush them off because they are toxic to breathe.)
- Let the wood dry.
- If some marking still remains, repeat the process.
- If only a light brown (tan) coloring remains, sand it out. It doesn’t go deep.
There’s no reason to neutralize the surface because oxalic acid is a relatively weak acid and the new finish you apply will keep water, which could activate any remaining acid, out of the wood. But if you decide to neutralize to be on the safe side, use baking soda or household ammonia reduced with a lot of water. Don’t use vinegar because acids don’t neutralize acids!