The Power of Oxalic Acid on Wood

When I started refinishing furniture, I was scared to incorrectly use chemicals, endangering myself or my family around me. Working with “real” chemicals would be far to hard. I simply did not understand the benefits of base chemicals like oxalic acid on wood. The skill to working with  chemicals is understanding their purpose and how to work with them safely.

It all clicked for me when I read The Furniture Doctor by George Grotz.  This book single handedly got me over my fear of truly working with wood and refinishing furniture.  It was like stepping into a wood shop with your grandpa.  Sometimes kind, sometimes comical and sometimes telling you to get over your fears by just doing.

Table of Contents

So, what is Oxalic Acid?

Oxalic Acid sounds dangerous.  Acid after all burn’s things, right?

While true, Oxalic Acid is a naturally occurring compound found in many the fruit and veggies that we eat.

When extracted and processed in its concentrated form, it becomes a white powder which you can easily purchase.  This powder is included in a lot of our daily use products which you have in your home right now. It’s found in some whitening toothpastes and many cleaning products.

Oxalic Acid in powder form
Wear Gloves! Be smarter than me.

How does Oxalic Acid Work?

Oxalic acid works like a type of stain remover or bleach. Depending on the situation and how it is applied, it removes stains or lightens the area it is placed on.

This is why Oxalic Acid is a god send to furniture refinishers for its unique ability to remove stains and lighten wood.

Using Oxalic Acid on Wood.

I saw several posts online recommending using Bar Keepers Friend on wood stains or to bleach grey wood back to its natural tones.  It is rather comical as its not Bar Keepers Friend that is producing the results, but the Oxalic Acid within Bar Keepers Friend.  Sure, you could opt to use Bar Keepers Friend, or you could opt to use pure Oxalic Acid which would be far more effective and probably cheaper.

When you add Oxalic Acid to wood stains, it works within the wood grain to eradicate the stain itself.  Does it work all the time?  No, it will depend on the type of stain and cause of the stain.

Wood Stained Grey prior to using oxalic acid
Chair after first use of oxalic acid on wood to clean stains

Before and after first coat of Oxalic Acid.  Not a huge difference.  It requires multiple uses for such tough stains.

What type of Stains does Oxalic Acid work best on?

Oxalic Acid works best on dark organic stains.  This includes, tannins, water stains, dirt, blood, rust, weathering etc.  If the stain was caused organically, oxalic acid has a good chance of removing it.

Furthermore, this wonder stain remover does not just stop at removing a stain, it will lighten the wood.  So those grey, weathered decks?  Oxalic acid is your secret weapon.

This does mean however, that if you wish to remove a stain, you may lighten that spot on your wood, causing a patch of discolouration.  To avoid this from happening, you would need to not only treat the stain, but the whole board which you are working on.

If your stains are not organic in nature, you will want to consider other cleaning options. 

Walnut Veneer with Oxalic Acid being applied to it
After 4 applications of oxalic acid there is little to no change in the walnut veneers colouring

As an experiment, I applied 4 applications of Oxalic Acid to walnut veneer.  As you can see there was little change to the deep colours in this walnut veneer. 

Safety Precautions; working with Oxalic Acid.

While Oxalic Acid is derived from plants which we eat, that doesn’t mean using it is risk free.  Since it is sold in a concentrated form, meaning it is stronger than the oxalic acid naturally occurring in plants.  It is toxic and would be poisonous to ingest.

Protect your Lungs

The powder when in the air is dangerous to breath in.  Breathing in the powder form of oxalic acid will make your lungs tingle and feel scratchy.  In the worst case, it can cause lung bleeding. It is important to always wear your respirator when working with oxalic acid or any other powder or fumes for that matter.

Protect your Eyes

In addition to being dangerous for your lungs, you should protect your eyes.  It causes your eyes to feel like they are burning and itchy.  Therefore, when you are mixing a batch, or working with oxalic acid you will want to protect yourself with your eye protection and if possible, work in a highly ventilated area.

Protect your Skin

Low concentrations may not cause any issues to your skin, but it will sting on any cuts on your hands.  Higher concentrations could cause skin irritations. I simply recommend always wearing gloves when working with the product.

While this long list of safety precautions may sound dangerous, it is like working with any cleaning solutions or bleach.  Proper safety is important in the wood shop.

How to mix Oxalic Acid

Mixing oxalic acid is rather straight forward and incredibly easy.  Just follow the safety tips.

Safety Tips

Mix Oxalic Acid in a glass jar or plastic container.  Never mix Oxalic Acid in a metal container! Oxalic Acid reacts with metal and should be therefore avoided. 

Wear your safety equipment when working with oxalic acid.  That is your gloves, eye protection and your respirator.

Materials Needed

Oxalic Acid tools needed for mixing a batch

Steps to mixing Oxalic Acid

  1. Add first warm water to your jar.   This can be however much you think you will require for the piece of wood you will be added it on.  
  2. Slowly add your oxalic acid powder. I usually add a tablespoon at a time.
  3. Stir between each new addition of oxalic acid (don’t use metal to stir!  I use a paint stick or wooden spoon).
  4. When the crystals stop dissolving into your water, you have added enough. The water has reached its maximum saturation of oxalic Acid. I tend to find that this is approximately 1-part oxalic acid to 6 parts water. 
  5. Your mixture is now ready to use.  
Oxalic acid added to water and dissolved
Fully saturated water with oxalic acid. It no longer disolves

(L) the water absorbs the Oxalic Acid. (R) the water is fully saturated and will not absorb any more oxalic acid.

How to use Oxalic Acid on wood: Step by Step

Applying Oxalic Acid on wood is incredibly easy.  The hardest part of the whole process is simply waiting for it to dry.

Materials Needed

Application Steps

  1. Clean the piece of wood which you will be working on.  You should always first complete a good clean before you take other steps in your refinishing process.
  2. Apply your oxalic acid mixture to your wood piece.  Apply with a sponge. Note: I have applied with a paint brush, but most paint brushes consist of metal which the oxalic acid will slowly destroy.
  3. Ensure you cover the board fully and equally to avoid any uneven lightening of the wood. If your wood is too wet, pat dry with rags.
  4. Allow to dry and see if the stains are light enough.  If not, repeat adding the oxalic acid and water mixture to the board until you reach the desired lightness, or the stains are removed.
  5. Once the stains are removed or you reach the right level of lightness, add water with either baking soda or cleaning soda to your wooden board.  This is to neutralize the oxalic acid still on the board.
  6. Then clean the whole board with warm water several times until you are sure that the oxalic acid crystals are no longer on the board.  Note: You can sometimes feel the grittiness of the crystals or see a white or sparkling residue. This could affect any finish that you add later if the crystals are not fully removed.

Next Steps

Water tends to raise the wood grain. You may wish to sand your piece lightly to knock back any grain that is now raised.  If you do so, wear your respirator.  Which you should normally wear it when sanding. But now it is even more important as you want to avoid breathing in any oxalic crystals which you missed in your clean up.

Shelf Life of Oxalic Acid

Powder or crystal form of oxalic acid will last indefinitely, if kept dry.  The mixed solution is another story.  Once you have mixed a solution of oxalic acid together, it begins immediately to breakdown.  It could last in a sealed jar, kept in a cool or cold place for probably a week or two.  After which I recommend getting rid of the mixture. 

Prior to disposing of the mixture, you could add baking soda to neutralize the oxalic acid or even dilute it further.

Is Oxalic Acid Environmentally Safe?

Even though high concentrations of oxalic acid to your skin, lungs and eyes could be damaging, it’s an environmentally safe product.  As previously mentioned, it is naturally found in the environment.

Adding baking soda to your mixture will also neutralize your mixture and make it safe for disposal at home. 

Examples of Oxalic Acid in Use.

I have used oxalic acid in several projects.  The most recent being used on vintage theatre chairs.  The thick, dark toned varnish and years of use had left the wood with grey staining.  I applied multiple coats of Oxalic Acid to lift as much of the grey staining out.  After which I followed up with a tan wash to brighten the chairs further and a topcoat for durability.

Another example of use is on a shelf in an antique cabinet.  The stains were removed quickly with only two coats of oxalic acid.

a Shelf in a cabinet with visible stains
Stains removed with oxalic acid in cabinet. The wood is visibly lighter

Same Shelf in a cabinet before and after 2 coats of oxalic acid.  You can see the patch of dark stains are gone and the wood is lighter in areas.  A sand and finish will even out the wood tones.

Frequently Asked Questions

Yes, you can, but not all woods will react the same.  Lighter woods will not lighten much further. And some dark woods will simply not lighten at all.  In these causes you would want to turn towards a stronger product like a two-component bleaching solution.

.

The higher the concentration the more stain removing power the oxalic acid solution will have.  I recommend using a concentration that is the highest saturation.

Keep adding oxalic acid to your water until the crystals will no longer dissolve.  This is the highest saturation you can reach.

 

Once beyond this level of saturation, you are simply adding crystals to the water which will not be absorbed.  This will not benefit you at all and is simply a waste of product

At a minimum until it is dried. You can also apply multiple coats until you get the desired affect you are looking for.  After which, then you can neutralize the oxalic acid and clean up. 

Leaving the solution on will simply result in the wood continuing to lighten over time, until the solution is ineffective.  There is no damage done to the wood itself.

You technically could.  Bar Keepers Friend, for instance, is a popular product to use to remove stains from wood.  I would, however, test any product you wish to use on a sample piece of wood.  That product could have other ingredients in it which react poorly with wood.  You will obviously want to avoid this.

Simply add baking soda to your mixture to neutralize the product. Once neutralized, then dispose of it down your drain and run extra water.

More Wood Working Products you need to know about

Final Thoughts

The thought of working with chemicals can be initially rather frightful.  However, with proper education and safety precautions, working with base chemicals can up your refinishing game.  Why search for products to “Hack” an effect when you can simply create and control the effect with the correct product?

Oxalic Acid is cheap, stores well and easy to use.  It can remove stains effectively that creates a huge improvement to that vintage furniture.  Up your refinishing game.

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Hey, I'm Sarah, the owner of Bold Wallflowers.

I'm on a self-taught journey through furniture refinishing and restoration, loving every experiment in my workshop.

Join me as I share my discoveries and gained knowledge with our vibrant community of fellow refinishers!

This post may contain amazon affiliate links. As an Amazon Associate I earn from qualifying purchases, if you decide to make a purchase through the provided links, at no additional cost to you.

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