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We have several large (3' x 6') blackened areas on our hardwood floor where water damage occurred. This discoloration goes deep. Our goal is to sand the floors and apply a clear coat, but we first want to know if there is a product that will remove the black stain. Also, if this cannot be done will a dark stain hide the problem?

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  • Wood bleach, AKA oxalic acid, follow all directions on the product exactly.
    – Gunner
    Commented Aug 25, 2012 at 18:38
  • Thanks. Will this product penetrate into the wood as far as the black stain goes, or just bleach the surface?
    – RET
    Commented Aug 26, 2012 at 5:15
  • 1
    Does it matter? All you see is the surface.
    – Dave
    Commented Nov 18, 2015 at 20:38

3 Answers 3

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+50

Removing Dark Stains With Sandpaper

  1. Remove the finish over the stain gently with sandpaper, moving with the grain of the wood. Use #100-grit sandpaper, and then feather the edges with #150-grit sandpaper.
  2. Sand the stain with #150-grit sandpaper, now that you have removed the finish. Feather the edges around the stained area with #0000 steel wool.

  3. Use tack cloth (lint-free cloth) to remove sanding dust.

  4. Put on several light coats of varnish to match the original finish.

  5. Feather the edges of the new varnish with #0000 steel wool to remove the slight bump between the old and new varnish.

  6. Wax the wood with a quality polish.

Remove Dark Stains With Bleach

  1. Bleach the wood with chlorine bleach if the stain turns out to be too deep to remove without excessive sanding.

  2. Don your rubber gloves and apply the bleach with a brush.

  3. Let it sit for a few hours. The stain should fade to nearly the wood’s original color, but it’s a slow process.

  4. Use a clean sponge and water to remove the bleach completely and prevent further fading of the wood color.

  5. Apply vinegar to neutralize the wood. This will prevent the wood from lightening the stain or varnish when you brush it on.

  6. Let the wood dry thoroughly.

  7. Apply wood stain, if needed, and let it dry again.

  8. Brush on several light coats of varnish to match the original finish.

  9. Feather the edges of the new varnish with #0000 steel wool to remove the slight bump between the old and new varnish. Remove dust with a tack cloth.

  10. Wax the wood with a quality polish.

Taken from: http://www.wikihow.com/Get-Water-Stains-Off-Wood

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The active agent in bleach is sodium hypochlorite. Normally, it will remove old stain color from wood but not ink stains or black water marks. For black water stains, use a saturated solution of oxyalic acid and hot water, then wipe with clean water.

The comment above which says to match the existing wood color with varnish etc., after stain removal, fails to mention that matching existing color can be extremely difficult. It's usually easier to refinish an entire top than to touch up a damaged one i.e., the long way is actually the short way.

Sand paper is usually a bad idea because it removes the surface of the wood, which is usually a different color than the wood immediately under the surface. Also very easy to sand through the veneer. Steel wool is a better way to go

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  • The problem was solved in late 2012.
    – RET
    Commented Dec 4, 2016 at 16:06
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Whelp, as a first-time homeowner with 1937 red oak floors-who had an elderly male dog and several black patches, I can only share what I experienced:

Wood bleach like oxalic acid will turn the surface of the wood so white it looks almost painted or shellacked in uneven nearly-translucent white, with the grain still unevenly showing through, a bit darker (reddish orangey brown in red oak). It did not penetrate enough to allow any sanding (and you need to do a little bit to prepare the surface so new clearcoats will stick) or we were right back to dark-gray-stained wood. Super harsh to the surface, and yet still didn't do the job!

I admit I followed directions but knew no more than what the label and YouTube videos showed. Maybe I didn't get all the floor finish off first to expose the bare wood, but I've read a few similar complaints elsewhere.

Sanding, even so deep you know there is a 1/4 inch deep cup-shaped divot in the floor (an experiment since we knew either way we'd have to paint or replace the damaged boards) only created a shallow bowl-shaped area of smooth dark-blotched wood, not as uniformly black because hard oak grain doesn't absorb any liquid evenly...but still damaged unevenly-gray wood. That's at least 1/3 the thickness of the original 1937 board itself, usually.

I didn't go any deeper; what would be the point? I have read and suspect that if a water or pee stain has sat long enough, or been wet multiple times, it will penetrate the boards so deeply you cannot sand it out. If you hit clean wood, it might be halfway through the board and you could reach the basement or subfloor without out ever finding clean wood.

So, you might be able to sand some, bleach small areas, and use stain pens to create a faux-wood pattern on the bleached white wood, then stain the wood to the same color as the other boards. This required using many thin layers to fine-tune the shade without going too dark--or careful mixing, testing, and mixing some more, on raw oak wood scraps (with a final drying to be sure) to match the color. I spent a lot of time doing single boards, then applying clear coating to the whole floor, after a light sanding...but it will never be perfect.

I found wood bleach to be the worst cosmetic solution. It looked like plastic with a reddish brown grain drawn on, and not that evenly white, either, so--dirty white wood-grained blotchy semi-glossy plastic. I ended up sanding that off the one area where I'd tried it, and sanding the other areas as best I could with a hand-sanding machine, allowing some very shallow-bowl-shaped divots.

I then used wood-touch up staining pens to very carefully continue the wood grain where necessary. Sanding deeply will expose a different grain pattern or perhaps it's wood that has not mellowed and soaked up varnish into the grain evenly over the years.

Regardless, there is an abrupt change after a deep sanding of one part of the board and you have to disguise it. I did this by "continuing the grain pattern" to the end of that board, as I am little bit artistic. It takes 2-3 pens, and patience more than talent. The water-based pens are best if the final clearcoat will be water-based, as the combination color/topcoat pens are oil-based ans so are incompatible with water-based top coats.

With a non-shiny satin floor finish, I ended up with fairly subtle divots you could still see when they had backlighting, and some faint gray areas...all in all not bad, considering. But the only real solution, I am sad to say, will be to replace the boards when the old dogs are gone and it's time to re-do the floors.

We'll have to strip the floors for a perfect match, or do a gentler sanding and just very carefully match the new raw boards to the old color as described above. It was not as hard as I'd thought, although I might have been lucky--and determined. Maybe for you, too? Caution: Do not go past the edges of each board with your staining to match, with the idea of "blending in". Individual boards DO vary, even more as decades pass and the wood grain becomes more pronounced. It would weird to have several adjacent boards be REALLY uniformly colored compared to nearby boards, even if the color was identical.

You must be patient with mixing stains...expecting the dry results to be about 2 shades lighter than wet stained wood. And you'll need to consider that oil-based coatings like oil-based polyurethane--but not water-based--yellow with time. Keep that extra goldish tone in mind when you consider the final result; it happens faster in sun-exposed areas.

I finally found a tint I could add to water-based poly to imitate that mellowing on those boards, so I did not have to strip the whole floor to the bare wood. The entire wood floor had only been done 5 years earlier. The tint I found was chemically compatible (I made sure) but meant for serious arts and crafts work. It only needed a tiny bit.

The ("cost is no object") floor experts say replacement is what you have to do as well. In the meantime, I am a woman who had no prior DIY experience and learned a lot. You can find an interim fix, too. Good luck!

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