What is oxalic acid used for?
Michael Dresdner: There are two common uses for oxalic acid. The first is to reverse the graying that happens to unfinished wood. Frequently, it is used to restore the original color to gray oxidized fences and decking. The other is to remove a particular type of blue-black stain caused when woods high in tannin come in contact with both iron and moisture. For that, you must first remove the iron that caused the problem. In both cases, flooding on a 10 percent solution of oxalic acid will reverse the problem and restore the original color of the wood. Let it dry overnight and wipe off the remaining white residue with a damp cloth. Once in solution it is fairly benign, but the concentrated crystalline powder, one form in which it is sold, is considered a toxic irritant to mucus membranes. It is, as its name implies, an acid. Keep it out of your eyes, nose and mouth with the appropriate dust mask and eye protection, wear gloves, and wash your hands after you are done working with it.