Rust-Stain Removal
When used carefully, this chemical compound is great for cleaning wood that has blemishes from rusty nails, concrete splatter, or water leaks.
Unfinished wood often gets stained from rusting nails, cement splotches or water leaks. Perhaps you’ve noticed this kind of staining around the corners of skylight wells that are lined with wood. If you need to remove such stains, try a solution of oxalic acid and water. Mix 1/4 lb. of oxalic-acid crystals with 1 gal. of warm water and stir until dissolved. Paint the solution onto the stained areas and let it stand for 15 minutes. Rinse the work with cold water, and repeat if necessary. Oxalic acid is sold at chemical-supply outlets, and it is poisonous. Be sure to wear rubber gloves and eye protection when you use it.
Ernie Alé, Santa Ana Heights, CA
Edited and Illustrated by Charles Miller
From Fine Homebuilding #29
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Oxalic acid is also in Bar Keepers Friend and I have found that works as well.
Oxalic acid keeps working unless you neutralize it. When I did boat varnishing the whole boat bubbled until someone told me this.Stain will "creep back" Water dilutes it but does not neutralize it. Hence it will keep eating the wood when covered over if it does not bubble the varnish. NEUTRALIZE WITH STRAIGHT WHITE VINEGAR! let it dry thoroughly after neutralizing before varnish or stain.
Neutralizing an acid (oxalic) with another acid (vinegar) just won't work- that's basic chemistry. See https://www.popularwoodworking.com/article/oxalic-acid-a-very-useful-bleach/
The link addresses the question directly. See also https://www.finewoodworking.com/forum/neutralizing-oxalic-acid
Both sources recommend a dilute solution of baking soda, which is alkaline.
A lot of times a simple solution of baking soda and water, or vinegar if it's a tougher "surface" stain works very well. That's all I've ever had to use. If it's worse than that, then it has penetrated very deep in the wood, and I can't believe any acid would remove that deep without destroying the wood, and causing problems with any type of wood treatment: stain, varnish, epoxy, etc. I'm no expert, but I'd sure be checking with someone who is before I covered very much surface with that type of treatment.