Can a dark stain on hardwood floor be corrected?

[Mary J.]

Hello! Recently picked up a multi unit with hardwood floors. In one apartment there’s a dark stain (I’m guessing something spilled years ago). I’m going to sand them, so was thinking of sanding and then staining with a darker finish to help hide it. Unless there’s some other special technique someone knows about? Thanks!

[Jim K.]

@Mary J.

Hate to be the one to tell you this, but the dark spot on the hardwood floor is very likely to be pet urine. Especially in oak hardwood floors (the most common) that were originally finished with wax, cat urine will eat through the wax and react with the tannins in the floor to create a dark spot. And it's very difficult to remedy.

Once you sand it, much of the stain will, theoretically, go away. But usually as soon as you refinish it, the stain will reappear. Once the cat urine gets in the wood, it's in for good.

There are typically two ways to work on a stain like this, wood bleach and oxalic acid. Wood bleach is a peroxide solution applied directly to the sanded stain. The most common application for this is a two-part mixed formula kit you can usually buy at a home improvement store. Alternatively, people use hair salon bleach cream developer with good results. You apply the stuff and cover it in plastic wrap. The only problem with this is that the peroxide bleach tends to really bleach the hell out of the oak grain pattern and if you're not right on top of the process, you often get an inconsistent light spot.

The second solution is to apply a solution of oxalic acid, the acid found in the cleaner Bar Keeper's Friend. But BFK is only partly oxalic acid: I like using the 100% stuff sold on Amazon. You mix this into a paste and apply to your floor, typically without plastic wrap. For my money, this usually provides a more natural-looking wood-lightening solution on oak. You can let it sit and not monitor it nervously.

Once you get the visible stain out of the sanded floor, you have to refinish the floor, and that's where things get funny. If you try to apply a heavy coat of oil-based polyurethane, shellac, or oil, you're going to to see the stain reappear again. What I tend to do is spray multiple very light coats of shellac over the stain over the course of a day. This does not darken the wood significantly. Once you've managed to spray four of these very light coats over the entire extent of the stain, you can apply polyurethane to the stain and it will not repenetrate, react with the cat urine still in the wood, and cause the stain to appear again.

There are other tricks that a flooring professional might use, like ripping up the existing floor in the area of the stain, fabricating a patch out of new oak, and darkening it up a bit with a stain to match the sanded surrounding boards. Staining the whole floor darker to match the stain is another way of doing the same thing. But this is usually far more expensive than it's worth unless the patch replaces wood that's been heavily damaged and is no longer able to serve as a functional floor.