How to use oxalic acid

How to use oxalic acid to restore stained and weathered wood

Black marks, water rings, and deep stains can take the shine off even the most beautiful piece of timber. These stubborn dark patches, commonly found on oak, teak, mahogany, and other tannin-rich woods, are usually caused by a chemical reaction between moisture, tannins, and iron. Sanding often won’t reach deep enough, and on veneers, it simply isn’t an option.

That’s where oxalic acid wood bleach comes in. If you’ve been looking for clear, expert guidance on how to use oxalic acid to restore wood safely and effectively, this guide will explain everything you need to know.

What is oxalic acid?

Oxalic acid is a naturally occurring organic compound that acts as a targeted stain remover for timber. It is particularly effective at eliminating black iron-tannin stains, deep water marks, greying and weathering, and rust contamination.

Unlike harsher two-wood part bleaches, which lighten the entire timber, oxalic acid wood bleach focuses on the stain itself. This means it restores the natural colour of the wood rather than bleaching it.

When should you use oxalic acid wood bleach?

Wood bleach oxalic acid is ideal when:

• Black water marks appear around taps or kettles
• Timber has greyed or dulled due to weathering
• Metal contamination (rust, nails, and screws) has left dark patches
• Veneer surfaces cannot be deeply sanded
• You want to restore the natural colour before refinishing

It delivers a cleaner, brighter, and more even tone without altering the character of the wood.

How to use oxalic acid

Oxalic acid wood bleach is a controlled substance and must be handled responsibly. Always wear gloves, eye protection, and a suitable mask, and work in a well-ventilated area. Avoid using metal containers or tools.

Prepare the wood

Wood bleach oxalic acid only works on bare wood, so any existing finish must be removed first, including:

• Varnish
• Lacquer
• Oils
• Wax
• Paint
• Previous stains

Once stripped, clean the surface thoroughly. Dirt, grease, or residue will prevent the solution from penetrating properly.

Mix your oxalic acid solution

To create the bleaching solution:

• Dissolve 2-4 tablespoons of oxalic acid crystals per litre of hot water
• Stir until fully dissolved
• Use glass, plastic, or ceramic containers - never metal
• For extra depth of cleaning, some professionals dissolve oxalic acid crystals in methylated spirits instead of water

This mixture becomes your working oxalic acid wood bleach.

Apply the solution evenly

For best and most consistent results:

• Apply the warm solution with a brush or sponge
• Treat the entire surface, not just the stained area
• Keep the surface damp throughout the process
• Allow it to dwell for 10 minutes to 1 hour, depending on the severity of the stain

Deep black marks may require more than one application.

Rinse and neutralise the wood

This step is essential for ensuring a clean, stable surface for finishing:

• Rinse generously with clean water
• Repeat until no residue remains
• Optionally wipe with a damp cloth to help neutralise
• If the solution was mixed with methylated spirits, rinse using methylated spirits, not water

Allow the wood to dry naturally for 24-48 hours. Once dry, the grain may feel raised. A light sanding with 150-220 grit will smooth the surface beautifully.

Tips for the best results with oxalic acid wood bleach

• Always test on an inconspicuous area first
• Avoid overlapping edges
• Don’t allow the solution to dry mid-process
• Repeat applications may be needed for deep stains
• Keep oxalic acid away from metal fixings where possible

Where to buy oxalic acid

You can buy oxalic acid wood bleach directly from our online shop and our physical store at B50 4JH.

Please be sure to read the Terms & Conditions before purchasing.

Frequently asked questions

Does oxalic acid bleach the natural colour of the wood?

No. It removes stains, not the wood’s natural pigment. The timber will return to its original tone.

How long should oxalic acid be left on wood?

Between 10 minutes and 1 hour, depending on stain length. Keep the surface wet throughout.

Should oxalic acid be neutralised?

A thorough rinse is typically enough. Neutralisation wipes help ensure no residue remains.

Can you use oxalic acid on finished wood?

No. Oxalic acid only works on bare timber. Any finish must be fully removed.

Is oxalic acid safe for all wood types?

Most hardwoods respond very well. Softwoods or unusual types should be patch-tested first.