Discolored(grey) hardwood floor in certain spots

[Eron A.]

Would you say this is something that could be fixed? I have no idea..

[Jim K.]

@Eron A.

For once, I get to tell my fellow 'Burgh investor @David Lee Hall, III he's partially wrong. This is a first, BP readers. Treasure it. It's not going to happen again soon.

This floor was originally finished with a wax paste, not polyurethane, and somewhere along the line people gave up on the regular rewaxing. Over time, the finish broke down

That is a normal wear pattern of filth tracked into the room and ground into the oak over time, and it was repeatedly washed with chlorine bleach, which further dissolved the wax paste helped along by sunlight, as David pointed out. The stain it is more obvious than it might otherwise be because, as @Mike Reynolds has also pointed out, in wood flooring installations you usually try to run the wood to and away from the light source, which didn't happen here.

Almost all of this stain is going to sand out. Some will be left, unfortunately. There are multiple ways to deal with the problem, but the easiest on this kind of floor is to mop the floor in an oxalic acid solution (oxalic acid is the key ingredient in Bar Keepers Friend but you can buy 100% crystals of it online through Amazon). So, sand the floor down, then mop the floor, let the solution sit on the floor for a day, mop it again, mop it again. Keep up with it until almost all the stain is gone. It will take a very heavy solution of the stuff and it will take time for the floor to dry out.

Now we get to the difficult part. Floors like this are finished today in oil-based polyurethane for heavy-usage purposes or water-based poly for show purposes, with a few wax paste lovers still around. Water-based poly looks nicer (it doesn't have an amber tint to it), but this floor's not doing to do well finished in water-based. So, go to HD or Lowes and get three cans of spray shellac.

Spray your first light coat of shellac on the entire floor standing up, from 36 inches away. You need to use an incredibly light touch, lighter than any spray-can finish you've ever used. You might choose to only spray the previously stained area, but spraying the whole floor on the first few coats will look better in the long run. Wait an hour. Spray a second coat. Wait another hour, spray a third coat. Then you start to get a BIT heavier, and over the stain only. Spray the next coat at 24 inches away. And the next.

If you overspray, you'll know, because the black outline of the stain will immediately start to come back, reacting with the shellac. But if you've got three, four, five very light layers of spray shellac over the stain, you can cover it well with progressively heavier coats without the wood turning color. I've seen $$$ restoration guys overseas do the shellac trick with airbrushes.

And once you're done with an undercoat of shellac, proceed with your three coats of oil-based poly. A lighter "regular" oil-based poly on the first coat, like the regular Minwax you might use on other wood projects, is better than thick purpose-blended floor-poly here. And that first coat of oil-based poly isn't going to be mopped around with an applicator, oh no, you're gonna be on your hands and knees with a white China bristle natural brush with no polyester blended in there. Purdy, to the best of my knowledge, no longer makes big block stain brushes in natural bristle, so the best available are the Wooster Bravo Stainers and even then you have to be careful because Wooster makes two kinds of stain block brushes, one they blend with poly and a significantly more expensive one that's all natural bristle.

Good luck, Eron.