Well, the Oxalic acid worked brilliantly for a day or three, but the
brown stains have reappeared over the course of two or three weeks. The
stains seem to be more "spread about" and less intense than they were
before.

We just re-treated with Oxalic again this weekend and again, the brown
stains all disappeared within a few minutes. I imagine that the stains
will come back, hopefully less intense again. This could be a longish
term project to gradually get rid of the stains.

I know this is about 6 years ago now, and the chances of anyone seeing this are slim, but did this eventually work for you? I am in a similar situation.