I was able to rescue some 100-year-old peroba do campo wood roof beams in quite good condition except for irregular black stains around holes made by old iron nails. How can I remove these stains?
Greg Williams: An oxalic solution may remove most of the color. Using touchup powders or background type touchup markers can cover any remaining color.
Michael Dresdner: Flood the wood with a 10% solution of oxalic acid, but apply it to all areas, not just the black stains themselves. Vacuum the residual salt the next day, or wipe it off with a damp cloth. Oxalic acid powder is considered toxic and is an irritant to mucous membranes in its dry state, so wear a dust mask when you sand treated wood and when you mix the powder into water initially.