Black water marks and other dark stains can be removed from wood using oxalic acid.
Black stains form when the natural tannic acid in wood comes in contact with moisture and iron. Since most water contains trace amounts of iron, tannin stains are a common problem in exposed wood.
Oxalic acid, commonly referred to as wood bleach, will lighten tannin stains without affecting the natural color of the wood. Oxalic acid will also lighten gray weathered wood and is often the active ingredient in deck brighteners.
Step 1: Remove any remaining finish on the affected area of wood. Use 100 grit sandpaper to sand the stained spot thoroughly.
Step 2: Protect the surrounding areas from spills and splashes with plastic sheeting or other waterproof material. Wear safety glasses, rubber gloves and protective clothing when working with any type of acid, including oxalic acid.
Step 3: Dissolve 2-4 ounces of oxalic acid crystals into 32 ounces of hot water. Hot water is key to the success of the project. If the solution cools before the job is finished, reheat and proceed.
Step 4: Use a scrub brush to apply the acid solution liberally to the darkened area of the wood.
Step 5: Allow the bleach solution to remain on the spot until the desired lightness is reached. Add additional solution as necessary to keep the wood soaked and to continue the lightening process. Do not allow the solution to cool.
Step 6: Once the dark spot is lightened, triple rinse the bleached area with clean cool water. The clean water will stop the bleaching process. Be sure to rinse the area at least three times to remove all traces of the acid.
Step 7: After rinsing, if the tannin spots still remain, repeat the wood bleaching process until the desired lightness has been achieved.
Step 8: Allow the wood to dry for several days before attempting to refinish. Once the wood is dry, lightly sand the area and wipe clean with mineral spirits.
Stain, seal or paint the exposed wood as desired.