I know it is off topic but I applied the oxalic acid to my floor last night. I had sanded about the thickness of a Loonie off the floor and the marks were way down in the wood and appeared like black stains near every nail and in some random places. The floor looked dirty.
I put it on all over and went back just after it was drying and painted the floor one more time with hot water.
the result is that it removed all the black marks and it lightened the floor somewhat It looks much better and I am pleased. It is sort of a "patina remover" . I wouldn’t recommend using it as a matter of course, but it sure changed the look of things and for the kitchen looks much nicer than a dirty looking floor. I think it would be helpful on a wood radio case where some part was not matching in color to the rest.