Black, moldy floor boards (under mat for dog’s water dish)

Q: I have hardwood floors.. I just changed a mat that I keep under my dog’s water dish. It is a waterproof mat, so I thought that moisture wouldn’t go through it. Anyway, lifted mat up and there are some black, moldy looking areas. I wiped, cleaned, used vinegar and water, but to no avail. Stains are still there.

Is there anything I can do to clean moldy wood floors? (I have prefinished hardwood that was laid on concrete slab.)

A: I think you will probably have to either refinish the affected boards or replace them. Not an easy thing to do, given that it is pre-finished. You might try oxalic acid to remove the stains but, if it works may also lighten the board considerably compared to the surrounding ones.