Oxalic acid is a naturally occurring compound found in plants such as spinach. In a natural form oxalic acid does not pose any serious risks to humans. It is when oxalic acid is cooked or processed that it becomes inorganic and toxic. The toxic version of oxalic acid is used as a bleaching agent on wood. You may buy oxalic acid at hardware stores, grocery stores and drug stores. Mixing oxalic acid is simple and takes just a few moments to complete. The strength of the oxalic mixture depends on the bleaching needs of the project.
Oxalic Acid Paste
Mix three parts oxalic acid crystals with one part warm water to create an oxalic acid paste. The paste is used on wood with dark stains created by water marks. The paste is used as a spot treatment and should not be used to cover an entire surface.
Apply the paste to the stained areas with a paintbrush and allow to dry.
Remove the oxalic acid paste with a wet sponge. Thoroughly clean or discard the sponge after removing the oxalic acid paste.
Oxalic Acid Wash
Create an oxalic acid wash to bleach larger sections of wood that do not require the deep bleaching the oxalic paste creates. For small areas mix 1 ounce oxalic acid with one cup warm water. For larger areas mix 8 ounces of oxalic acid crystals with one quart warm water.
Apply the wash to the wood surface using a sponge. The wash will bleach the surface of the wood evenly. It is important to cover all areas of the wood to achieve the desired result. Be certain to get the wash in trim pieces and into corner pieces.
Remove oxalic acid wash with a clean sponge and clean water.