Introduction: Removing Black Stains in Wood Furniture With Oxalic Acid

Iron oxide stains in wood are usually dark spots caused by prolonged exposure to water and / or metal. The iron oxide reacts with the tannins in the wood and stains it black. Wood can intentionally be stained black in the same way, and that's called Ebonizing.

Along the edge of my desk I kept a rusty metal ruler, and sometimes I would over-water the plant on the desk and the water would pool up under the ruler. In a short time this lead to black stains all along the edge of the desk.

I've seen some amazing results from furniture restorers using oxalic acid to remove such stains, but I couldn't find it locally and never got around to ordering it online.

One day I came across a How-To by Thomas Penrose and realized that I already had oxalic acid in the form of Bar Keeper's Friend. A big thanks to him for the write up. I'm going to relay those basic steps and add some tips of my own.

Materials and Supplies:

  • Bar Keeper's Friend or similar powdered metal polish containing Oxalic Acid (Red Bear, etc.)
  • Water
  • Paper Towels
  • Chip Brush (optional)
  • Plastic Paint Scraper
  • Stiff Nylon Brush (optional)



Step 1: Assess the Stain

If the stain is dark, almost black, chances are it's from water exposure, but it could also be from ink etc.

If the stain is dark red, it's probably from wine or food or anything else.

This method will only work to remove dark iron oxide stains. It won't hurt to try, but it'll probably be more effective to bleach / sand / refinish other types of stains.

If the wood has a polyurethane finish or other surface coating and you see a stain, it's stained underneath, and that coating needs to be removed first.

If the piece has an oil finish or if it's raw wood, you don't need to do anything to prep the surface.

Step 2: Make a Paste With Bar Keeper's Friend and Water

Add some water to Bar Keeper's Friend until you have a consistency that you can work with.

The ratios aren't important, but you don't want it to be too wet. Excess water is just going to raise the grain. All you need the paste to do is sit on the surface of the wood. You don't need it to soak in.

Step 3: Wipe It On

Always test on an inconspicuous area first!

  • There shouldn't be any negative effects, but it's good to find out on a small area instead of the whole surface if there is.

For the first application, target the stains.

Wipe or brush it on and then wait until it dries (15-30+ minutes).

As it's drying, you should see the white paste turn slightly yellow. This is good.

Step 4: Scrape It Off

I scraped the dried mixture off and vacuumed it up rather than wetting the surface and wasting paper towels.

After the first application you should already see a huge difference. If not, then this method probably isn't going to work on the stain.

Step 5: Second, Third, Fourth Applications

Mix up another batch and apply it the same way you did before.

When the stains are mostly gone, you'll notice that the area you cleaned will be lighter than the surrounding areas.

For the next application apply the mixture to the entire surface of the piece to help blend it together.

Step 6: The End Result

I only needed 3 applications to get the stains to a point where I didn't mind anymore. Further applications will remove the stain completely. My plan is to refinish the top of the desk at some point, first by applying oxalic acid to any other stains, and then rubbing with #0000 steel wool and oil to clean the dirt while leaving most of the patina that's built up since I've owned it.

Final Tips:

  • Because Oak has a very open grain, a lot of the mixture will be stuck in the grain of the wood. I used a stiff nylon brush in a circular motion to remove it. This probably isn't necessary on other species of wood.
  • If your refinishing process involves steel wool, be really careful to remove all of the metal shavings from the surface, especially with Oak. It's recommended to use brass wool or synthetic wool to avoid this. When the metal particles get wet in the future, it will just start the whole staining process over again and you'll have to redo it. In practice steel wool is fine, just make sure to blow the piece off with compressed air and buff it with a microfiber to remove all of the shavings.
  • When sanding, don't sand too deep. As you cut further down into the surface you might expose more of the stain, and then you'll have to reapply oxalic acid all over again.

Comments

author
newberries made it! (author)2017-04-07

This worked great, and fast! Plus, I already had Barkeeper's Friend in the house. Finished it with Minwax Wood-Sheen and it turned out beautifully.

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author
dlager (author)2016-09-28

Wish I had seen the lemon juice idea first. A lot of powdery residue got stuck in the oak grain of the table leaves I was working on (not the main table top for some reason). Maybe it was because the grain on the leaves was more open due to stripping off the varnish right before applying the Bartender's Friend. I plan to continue trying mineral spirits or water and a stiff brush to dissolve or brush it out. It is not coming out easily. If I can't get it all out, will it cause a problem with staining and varnishing?

author
dicandoit (author)2016-06-11

Thank you! Helpful advice! Does anybody know how to get Chocolate wine off a log home cedar wall? I bought it this way, I didn't get to drink any of the Chocolate wine!!

author
DRAGUA1979 (author)2016-06-06

Follow the grain with the brush as well so you don't tear up the wood.

author
sunspotbaby64 (author)2016-06-05

regular clorox bleach will remove the stain.No matter what you use though,you will have ti restain the the wood.After bleaching the stain damage,you need to rinse the wood really good with water.Let dry a few days.Sand and then stain.Then put on a sanding saler,sand,then apply two finish coats of polyurethane

author
Evelyn348 (author)2016-06-05

timely advice, I have wooden clothing racks for drying clothes outside and was wondering what I would need to do.

author
ooohlaa (author)2016-06-05

we all have to remember to choose the least toxic method for all approaches to life's problems, i.e., start with lemon juice ... it might take a few doses but better not to breathe fumes of anything, we already do from outdoor air, to public bathrooms, to department stores, to golf courses etc. we are always breathing in toxicity. that being said I would use this method and am grateful for it, if must be! thanx

author
foobird (author)2016-05-27

Take care when using Oxalic acid. It is poisonous.

author
momoluv (author)2016-03-28

I didn't even know those stains could be removed! Great job :-)

author
Tmotj (author)2016-03-28

I've been a high end tile setter for over 35 years Lemon juice works to take the black off of raw oak...I learned it the hard way, the stains shown look to be the same type, just wipe pure lemon juice on the area and let it dry

author
acheide (author)2016-03-28

Good Instructable. I used this method once for water stains on cedar ceiling boards.
Thanks.

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