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How to remove stains and marks from wood furniture with OXALIC ACID

Furniture Restoration Help & Advice

The best way to remove stains from wood furniture is by using oxalic acid. Professional furniture restorers have been using this very safe and easy to apply treatment for decades. Oxalic acid is very effective at removing food and drink stains, including spilled red wine and water marks. In this easy to follow article, I describe how anyone can use oxalic acid to remove unwanted marks from their wooden furniture.

Note: *This process will only work on bare untreated wood

What is Oxalic Acid?

rhubarb contains oxalic acid

Oxalic acid is a naturally occurring acid. It can be found in rhubarb, spinach, and oxalis (a type of weed with a bright yellow flower). In its pure form, oxalic acid is toxic and can pose a danger to humans and animals. Most available products are diluted and present a low level of risk, however care is still needed when using any product containing oxalic acid and we advise using gloves and a mask during its application.

Why Do We Use Oxalic Acid To Remove Stains From Wood?

Oxalic acid has many uses around the home and can be used to remove stains from metal and stone as well as wood. In our YouTube video  "How to Remove or Lighten Stains on Wood using Oxalic Acid", you will see that I use it to lighten and remove staining from oak and mahogany as well as to pull through the natural colour and grain as a way of reviving the wood.

ink-stained oak

Oxalic acid is particularly effective at removing the stains produced by metals, specifically nails. Often, the tannins contained in certain species of wood react with the iron in nails, producing dark stains which can only be removed with oxalic acid (oxygen and chlorine based bleaches are less effective). Oxalic acid makes most insoluble iron compounds soluble and therefore removable as they can be washed away.

For the purpose of lightening or removing unwanted marks and stains, to bring the colour of the wood to the surface, oxalic acid has to be applied to bare wood. It will not work on a finished piece of furniture. As I demonstrate in the video, you would need to strip off the finish in order for this process to work.

How Does Oxalic Acid Work?

In my professional experience (I’m not a chemist), Oxalic Acid works by reacting with the natural tannins in the timber. When first applied you can see for yourself that it starts to work almost immediately. It does this by pulling the original colour of the timber through to the surface.

So it is working like a bleach but instead of bleaching the wood from the surface down it works in the opposite direction by pulling the colour up and out. This is why it is so popular with furniture restorers.

Using Oxalic acid is very different from using two-part wood bleach. (known as A-B bleach) which is used to intentionally bleach white, the fibres of the wood from the surface down. (Hydrogen Peroxide & Sodium Hydroxide)

I have never used any of the hacks that I regularly read about involving baking soda, toothpaste and vinegar (acetic acid). In my opinion they are best left for cleaning your teeth and baking cakes.

17. toothpaste

When Not To Use Oxalic Acid

In some situations we would not use oxalic acid on an unusually very thin veneer. This is because if the veneer is so thin it would not retain much of its original colour for the oxalic to work on. This is quite a rare situation, most veneers will work nicely with oxalic acid. 

How To Use Oxalic Acid To Remove Stains From Wood

In our Youtube video "How to Remove or Lighten Stains on Wood using Oxalic Acid", you will see that I first demonstrate how to use oxalic acid to lighten and remove stains on a piece of solid oak. We often will use it as a wash, over a stripped piece of furniture to refresh the natural colour of the piece. This is a great way of subtly refreshing the wood naturally without using stain.   

Using oxalic acid to remove stains from wood

You Will Need:

  • Oxalic acid crystals
  • Soda/washing crystals
  • Hot water
  • Sponge scouring pads
  • Two containers
  • Protective gloves & face mask
7 scouring pads for application of oxalic acid

Method

Personally I very much prefer mixing my own oxalic solution. It allows you to control the strength of the mix and any unused crystals will last for years in a sealed container. 

  • Dissolve one heaped dessert spoonful of oxalic crystals into 150ml of hot water
  • Repeat this process with the soda crystals in a separate container.
  • Put on protective gloves before soaking one sponge in the oxalic acid and apply it to the stains using the rough side. You can ‘spot apply’ the acid in this way quite liberally before coating the rest of the wood as we intend to sand the surface when dry. This will rid the wood of any tide marks produced by the spot application. Avoid the creation of tide marks by ensuring that the surface does not dry out before neutralising with the soda crystals.
  • With a clean sponge, apply the soda crystal solution. This appears to catalyse the reaction and neutralises the acid stopping the bleaching process.
  • You can repeat steps 3 and 4 several times if you wish to before allowing the wood to dry for 24hours.
  • Once the wood is dry, put on a mask and lightly sand the surface of the wood to further remove any evidence of the treatment.
8 fillling the container with hot water
6. oxalic and soda crystals
washing soda reaction

The second demonstration in the video shows how to use oxalic acid on a piece of mahogany in order to revive what turns out to be a beautifully coloured and grained piece of wood.

sanding bare mahogany

The same process is repeated with a slightly more concentrated solution of oxalic acid and soda crystals (1 heaped dessert spoon into a container dissolved with 100ml of hot water).

I choose not to spot treat any marks in order to avoid any tide marks, as I don’t want to sand the veneer on this piece of mahogany. After completing steps 4 and 5, I wipe the mahogany with a dry cloth to remove any excess solution and allow it to dry for 24 hours. Wearing a mask, I then lightly sand the dried surface with 320 grit sandpaper.

What about spot removal of stains and marks? 

You can if you wish to accurately bleach a specific area using a small artist's brush. Very carefully brush the oxalic solution on to the stained area. Take great care doing so, This method can be very effective at removing a few single unwanted marks without bleaching the rest of the surface. 

11. spot detailed bleaching

The Finishing Process

The final part of this tutorial shows how you may like to finish the wood with French Polish in order to really bring out the revived colour and show how well the oxalic acid has performed.

You Will Need

open-weave pure cotton buffing cloth

Method

  • Create a palm sized, pear shaped fad with the open-weave buffing cloth.
  • Charge the fad with methylated spirits 20% and Shellac 80 % (approximately) but don't over soak it.
  • Force the fad onto the onion board (or smooth non absorbent surface) : This helps to evenly distribute the polish mix into the soft cotton, so it acts as a reservoir. It also flattens the sole of the fad so it has a smooth flat surface to apply the polish with.  
  • *You can alternatively apply the polish mix by using a polishers mop. But on a large flat surface fadding is more effective. 
  • Using long, even strokes, apply the fad to the surface of the wood in the direction of the grain in order to bring out its revived colour.
  • Leave to dry for 24 hours or longer before choosing to wax with Gilboy’s Gold Beeswax polish.
making a fad
Gilboys beeswax wood polishes

Video: How to Remove or Lighten Stains on Wood using Oxalic Acid

Did you Know? Oxalic Acid Keeps Bees Safe

A relatively recent discovery by beekeepers is the use of Oxalic Acid in treating honey bees against the devastating effects of the Varroa mite. It has been very effective at protecting the bee colonies and is a food safe treatment when applied correctly. 
Ref: Bee scientists to force killer mites to self-destruct

About the Author: Simon Gilboy

Simon's career began in furniture restoration in 1987.  Leaving school at 16 and signing on as an apprentice French Polisher at Staverton joinery, he has accumulated over 30 years experience in the restoration of fine and antique furniture.
In 1994 Simon opened the doors on the first Gilboy’s workshop at the Riverside in Staverton with financial and mentoring help from The Prince’s Trust.
In 2015 after years of searching for a beeswax furniture polish that would befit the fine furniture Gilboys were restoring, Simon developed his own beeswax polish using only the very best of responsibly-sourced ingredients.
Simon says, "My intention was not to compete with anyone on price, but to simply make the best beeswax polish it was possible to make"

You can usually find Simon in the Gilboys workshops filming instructional how-to videos for the Gilboys YouTube channel, on help forums, or actively finding new ways to preserve the past for the future.


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