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

Author: Simon Gilboy | Updated: 30th November 2023

This straightforward guide explains how to use oxalic acid to effectively remove undesirable marks from your wooden pieces.

**Please note, this method is only suitable for bare wood.

To rejuvenate the natural colour of wood by lightening or eliminating unwanted marks and stains, it’s essential to apply oxalic acid to the bare wood. This approach is ineffective on finished furniture. In such cases you must strip the finish from the furniture before the treatment can be effective.

Oxalic acid is an exceptionally effective solution for removing stains from bare, untreated wooden furniture. Widely used by professional furniture restorers for its safety and ease of use, it is particularly adept at eliminating a range of stains, including those from food, drink, red wine, and water marks. It is particularly effective at removing stains caused by metals, such as those from nails. In certain types of wood, tannins react with the iron in nails, creating dark stains. Oxalic acid excels in these situations, turning normally insoluble iron compounds into soluble ones, allowing them to be easily washed away.

When Not To Use Oxalic Acid

In certain rare instances, it's advisable to refrain from using oxalic acid on extremely thin veneers. The reason for this caution lies in the minimal thickness of the veneer, which may not hold enough of its original colour for the oxalic acid to effectively work on. However, this is an uncommon scenario, as most veneers typically respond well to treatment 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.   

You Will Need:

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.

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.

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. 

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

• Special/transparent pale polish (shellac)
• Methylated spirit (denatured alcohol)
• Open-weave buffing cloth or cotton wadding 
• Onion board
• Gilboys Beeswax Polish

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.

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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