Black stains on wood floors not fully sanding out

Q: We have discovered many black stains on wood floors after we pulled out the carpet. We tried to sand them out. While some got lighter, others did not. There seemed to be a smell of urine during the sanding, so I’m assuming that these stains may be from pets.

Is there anyway to get these black stains out?

A: Your best option is to replace the damaged boards, which, if these pet stains are all over the place, would mean replacing the floor. Option 2 is to stain the floor dark enough to mask the spots. If odour is a concern, you could apply a barrier coating over the stain before applying the finish top coats. Such products as Zinsser Seal Coat or Dura Seal Universal Sealer are dewaxed shellac products that are excellent barrier coatings which dry very quickly and offer excellent adhesion.

Related Q: I am into another refinishing project and this time I dove in, as usual. After we pulled up the carpet we discovered some surprises: two 3 x 2 ft. patches of plywood and some dark stains that I thought might sand out. They did not. Do you have any experience with this?

I replaced the two plywood spots with some left over white oak from my last project that I ripped down to 1 1/2 in. x 3/4 in. and some light colored red oak boards that I ripped and grooved. My house is 50-60 years old and has that really narrow oak which is not available anywhere. The patches look OK, not great. Do I have any options besides replacing boards in the stained areas? Like (very dark) staining the floor?

A: How narrow are the boards? You should be able to find down to 1 1/2. I have seen boards even narrower but very rare. I have used a commercial bleach years ago which came in part A & B. Quite nasty stuff. I don’t even know that I could buy it now. I’ve heard something about peroxide removing stains. That is something to look into. I am going to check Lee Valley Tools and see if they sell anything that will remove stains. If not, the only options are to replace the damaged areas and/or stain the floor dark. You can buy oxalic acid online.

Related Q: I noticed darkening on the edges of two of my oak floor boards, from liquid. How can I get it to lighten?

A: You can apply oxalic acid, which may take more than one application. I’ve heard you can get this at the pharmacy.

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