Instructions for Using Oxalic Acid for Bleaching Stains in Wood

Safety Precautions
Oxalic acid is highly corrosive and toxic in its refined form; use rubber or vinyl gloves whenever you handle it. Hazardous gasses may cause health problems, so always mix and use it in a well-ventilated area. The crystal and sandpaper dust is potentially dangerous; use a dust mask and eye protection. Have a pail of rinse water available to flush your eyes or to wash away spills if needed.
SuppliesOxalic acid crystals are available at most hardware stores; in addition, you need white vinegar, 1 or 2 large dry paintbrushes, 2 or more large clean sponges, a small plastic mixing bowl, a large plastic mixing bowl, a mixing stick, a plastic pail of hot water, a pail of rinse water, gloves (rubber or vinyl), goggles or other eye protection, a dust mask, a drop cloth, several clean rags, extra fine sandpaper (280 grit at least) and a tack cloth.
ProcedureBegin by stripping old varnish, which may reveal stains. Use mild bleach on ink and chemical finish stains and oxalic acid for water stains, such as black spots and dark rings.
Use drop cloths and sand away oil or remnants of varnish; sand in the direction of the grain. With a dry paintbrush, whisk away the dust, followed by a thorough rubdown with a tack cloth over the entire surface.
Mix three parts oxalic acid crystals with one part hot water in a small plastic bowl to make a paste. Apply the paste to the stained areas with a brush or sponge, avoiding contact with any metal. (Note: if the wood has been previously stained to a darker color, you have to apply the paste to the entire surface). Let sit for 3 to 5 minutes for soft woods and for over an hour on hardwoods; if the crystals dry, either remoisten with a couple drops of water, or reapply. Once the desired color is reached, remove the paste with a sponge.
Combine 1 oz. oxalic acid crystals with 1 cup hot water in a larger plastic bowl to mix a “wash” and coat the entire surface to reach an even color. When the item is consistently bleached, use a clean sponge and warm water to wash away all of the acid.
To counteract the acid still saturating the wood, dilute ammonia with water 1:1 and use a clean sponge to wipe the entire surface. Immediately rinse again with clean water using a sponge and let the furniture dry for two days.
Once the item is dried, use extra-fine sandpaper to gently sand the entire piece. Remove the dust with a clean, dry paintbrush, followed by a thorough rubdown with a tack cloth.
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