How to use oxalic acid on bare wood

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, protective eye-ware and a mask during its application.

Wear protective clothing: Rubber Gloves, Face Mask and Safety Glasses.
Keep away from children.

We have found in the workshop that oxalic acid is most effective when applied to oak, walnut, mahogany and teak:

• The solution can be applied directly to the stained area for spot removal
• Or it can be used as a general wash over the entire surface
• Or you can use both of the above techniques, by spot bleaching the affected area firstly and then once you are happy with your spot bleaching you can wash the solution over the surface as a general revive of the whole surface

Method

1. Apply the hot solution to bare wood with a brush, sponge or cloth to help remove unwanted stains and revive the natural colour of the wood
2. Ensure you rinse the surface thoroughly with clean fresh water to remove any excess oxalic acid. If you don't properly wash off the treated area, there is a strong chance that it will react with any newly applied finish: Preventing drying of the finish and or blistering of the finish
3. If you are trying to avoid raising the grain of your wood surface you can alternatively use methylated spirits to dissolve the oxalic crystals. The advantage being a quicker drying time for finishing
   

Can you use oxalic acid on veneer?

Yes you can use it on veneered and solid wood furniture

*Trade Tip* - Soda Crystals
A neat trick I often employ when using oxalic as a reviving wash:

Neutralising the oxalic with a hot solution of soda crystals

1. Mix the soda to the same ratio as the oxalic 10:1 in very hot water.
2. Apply the soda solution shortly after you have applied the oxalic and you will see it react immediately.
3. You can repeat this process but its effectiveness will lessen with each process

**We do not guarantee by using this product it will remove all stains. Some wood species can be resistant to the bleaching process of oxalic acid in solution.

x1. 100g tin will make approximately 1 litre of oxalic acid in solution @ 10%
x1. 100g tin will make approximately 2 litres of solution @ 5%

*One level teaspoon =  approx. 5 grams + 100 ml of hot water makes a 5% solution

Material Safety Data Sheet (MSDS) Click here to view or download

Here are the top 5 frequently asked questions regarding the use of oxalic acid with wood:

What is oxalic acid, and what is its purpose in woodworking?

It is a naturally occurring organic compound that is often used in woodworking for its bleaching and cleaning properties. It is particularly effective at removing stains caused by water, rust, tannins and at lessening the marks left by ink pens, coffee and tea.
Oxalic acid will often help restore the wood's natural colour prior to refinishing.

How do I safely use it on wood?

When using oxalic acid, it's essential to take safety precautions since it can be harmful if ingested, inhaled, or if it comes in contact with the skin. Use gloves, goggles, and a dust mask when working with oxalic acid. To use it on wood, dissolve the oxalic acid crystals in hot water according to the manufacturer's instructions. Apply the solution to the stained wood using a brush, let it sit for the recommended time (usually 10-20 minutes), then rinse thoroughly with water. Allow the wood to dry completely before refinishing.

Can it damage wood?

When used correctly, oxalic acid is safe to use on wood and should not cause any damage. However, excessive use, incorrect dilution, or prolonged exposure can potentially lead to the wood becoming discoloured, weakened, or damaged. Always follow the manufacturer's instructions for proper dilution and application to minimise the risk of damage.

Will it remove all types of stains from wood?

It is particularly effective at removing stains caused by water and rust. It also can be very effective at removing the other stains as mentioned above, but it’s not guaranteed to work. You may need to seek professional help with removing stubborn stains and marks 

Can I use it on all types of wood?

It can be used on most types of wood; however, it is most effective on woods with high tannin content, such as oak, teak, walnut, or mahogany. On woods with low tannin content, the bleaching effect might not be as pronounced. It's essential to test the oxalic acid solution on a small, inconspicuous area of the wood before applying it to the entire surface to ensure that the desired effect is achieved and to avoid potential damage.

Here are the top 5 common misconceptions about oxalic acid in terms of its use in restoration:

Oxalic acid is a universal stain remover:

While oxalic acid is effective at removing certain types of stains, such as those caused by water and rust, it is not a universal stain remover. It may not effectively remove 100% of other types of stains, such as grease, ink, or dye, but in our experience it certainly does greatly lessen them. Some stubborn stains may require specific cleaning methods or more aggressive professional wood bleaches.

Oxalic acid can bleach any wood type evenly:

Oxalic acid is more effective at bleaching woods with high tannin content, such as oak, teak, walnut, or mahogany. Its bleaching effect may not be as pronounced on woods with low tannin content. Therefore, it's essential to test the oxalic acid solution on a small, inconspicuous area of the wood before applying it to the entire surface to ensure the desired effect is achieved.

Will it restore colour to sun-faded bare wood:

Oxalic acid primarily works on stains caused by water, rust, or tannins and may not be so effective in restoring the colour of wood that has been faded due to sun exposure.
However, saying this, again in our experience we have often found it to be very effective at restoring the natural colour of the timber. Sun-faded furniture and wood often requires a combination of actions, such as sanding, treating with oxalic and refinishing. 

Can I use it as a wood finish:

Oxalic acid is a cleaning and bleaching agent, not a wood finish. It can help remove stains and revive the natural colour of the wood, but it does not provide any protection or enhancement to the wood's appearance. After using oxalic acid, the wood should be rinsed, dried, sanded and refinished with an appropriate wood finish to provide protection and enhance its appearance.

Oxalic acid is harmless:

Oxalic acid is a potentially harmful substance if not handled correctly. It can cause skin irritation, eye damage, and respiratory problems if inhaled. It is crucial to take safety precautions when working with oxalic acid, such as wearing gloves, goggles, and a dust mask. Always follow the manufacturer's instructions for proper dilution, application, and disposal.

Is it the best stain remover for wood?

Oxalic acid is considered one of the best stain removers for wood due to its effectiveness in removing specific types of stains, such as those caused by water, rust, and tannins. Its benefits include:

Targeted stain removal: Oxalic acid has a unique ability to dissolve and remove iron and tannin stains from wood. This makes it particularly effective for treating black or dark stains caused by water, metal, or high tannin content in the wood.

Wood colour restoration: Oxalic acid can often help restore the wood's natural colour by neutralising the compounds responsible for the staining and pulling the natural wood colour to the surface. This makes it a very useful tool for preparing wood surfaces for refinishing and enhancing its natural appearance.

Compatibility with various wood types: Oxalic acid can be used on a wide range of wood types, although its effectiveness may vary depending on the wood's tannin content. It is especially effective on woods with high tannin content, such as oak, teak, walnut, or mahogany.

However, it's essential to note that oxalic acid is not universally effective on all types of stains, and it is not necessarily more eco-friendly than other options. Here are some points to consider:

• Limited effectiveness: Oxalic acid is not as effective at removing stains caused by grease, ink, or dye. Different stain types require specific cleaning methods or  specialised, professional wood bleaches.
• Environmental impact: Although oxalic acid is a naturally occurring compound, it can be harmful to the environment if not used and disposed of correctly. Large quantities of oxalic acid can be toxic to aquatic life and can harm vegetation if it enters waterways or soil. It is crucial to follow the manufacturer's instructions for proper dilution, application, and disposal to minimise environmental impact.
• Safety concerns: Oxalic acid can be harmful to humans if ingested, inhaled, or if it comes in contact with the skin. When working with oxalic acid, it's essential to take safety precautions, such as wearing gloves, goggles, and a dust mask, to minimise health risks.

In summary, oxalic acid is highly effective at removing specific types of stains from bare wood, but it is not universally effective.