After the finish was applied, dark spots appeared

Q: Some friends of ours had their hardwood floors refinished by professionals. The floor looked perfect after sanding, before the finish; but, after the finish was applied, dark pet spots appeared. I am refinishing my floors and I don’t want the same thing to happen.

My sanding is complete and floors look good now, but is there the chance that the same will happen? Or was something done incorrectly to cause this to happen to our friends floor?

A: There is no way “pet stains” can be sanded away and disappear, then re-appear. They are either gone or they aren’t. If they aren’t, you will see them on the freshly sanded wood.

Perhaps these black marks are the result of “tannin pull”. Occasionally a floor, particularly white oak, can have high levels of tannin which can react with the floor finish and leave black marks. I’ve only seen it a handful of times in 34 years.

Similar Q: My friend had her floors refinished and some dark spots have appeared in the days that followed. Any idea what can cause this?

A: Wild guess? A tannin reaction in the wood or there is a moisture problem under the floor.

Follow-up Q: I looked at the bottom of the floor from the basement and didn’t see any moisture. Do you know of anything that can be done to fix it?

A: If it was a tannin reaction, the spots would need to be sanded out and a sealer applied first that is designed to help prevent this. It depends the type of finish that was used. If it is an oil based coating, a universal sealer (de-waxed shellac) could be used. If water borne, then a water borne sealer.

Besides sanding the dark spot out, I don’t know. Maybe just the board(s) involved could be sanded, taping off around the board to isolate it from the ones beside it. Then coat it and remove the tape.

Similar Q: After removing carpet I noticed there were some dark spots in the wood. I sanded the floors, but the spots are still there (a little lighter than before). what can I do to these areas before I stain the floor?

A: Either change the boards or if you think the stain is dark enough, just stain and finish which should hide the dark spots.