black spots on wood pinterest pin

Black Spots on Wood

Sarah Stahl
Sarah Stahl
Passion for transforming tired, old furniture into bold works of art. Sharing my learning with you.

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Discovering black spots on your beloved wood furniture can be disheartening.  While these unsightly marks can stem from various sources, they are most frequently attributed to water damage, a common adversary in the world of furniture refinishing. In this post, we’ll delve into the mysteries behind these dark blemishes, offering insight and solutions to restore the beauty of your treasured pieces.

Table of Contents

What causes black spots on wood?

While black spots on wood are usually caused by water damage, they can also be caused by a mix of water and iron, mould or even burn marks.  Having an idea what caused the spot can help you decide how to treat the spot.

Water Damage

When water penetrates wood, it can cause the wood to discolour, over time going grey or even black.  Think a plant pot left on a wooden cabinet. The condensation or leaking of the plant pot on the wood will leave a ring of damage.

This is your standard dark circle or ring of damage.

Black spot on wood caused by water damage

Iron Oxide

I have personally experienced these types of black stains in a huge learning moment for myself.  It was caused by using steel wool on raw walnut veneer, with water.  I don’t recall what I was trying to accomplish, but the result was tiny black dots all over the walnut veneer and total panic for me.  Quick research taught me that when metal reacts with tannins in the wood, it can result in black spots.

These types of spots can be in the shape of the metal item placed on the wood.  Think a metal cola can that was wet.  Nails left on wood in the rain, etc…

Mould and Mildew

Wood or furniture left in water for a long time, or in a humid area could experience mould or mildew.  You can normally feel the mould or mildew as it can sit on the wood itself.

However, in some cases, it will penetrate deep within the wood grain.

Burn Marks

Burn marks on furniture tends to be cigarette or candle related.  Most often it will look like  chard, black wood.  The edges can be flaky and the centre of the burn can be indented.

Burn mark on wood

Repairing Black Spots on Wood Damage

Before we dive into how to treat black spots, lets look at recreating these spots for treatment.   At this moment I did not have any pieces with water, mould or burn damage.  As I come across pieces with these types of damage, I will document my repairs.

Recreating Black Spots caused by Iron Oxide

I recreated in an experiment the time I caused iron oxide caused by water and metal. 

Using steel wool and water, I rubbed both walnut veneer vigorously until black spots started to appear as it dried.  Both veneers immediately started developing tiny, black dots and the veneer took on a darker, cloudy look.

Creating black spots on wood with water and steel wool

Does your piece have a finish on it?

It is important to note that these treatments were done on raw veneer without any finish. I would always recommend removing the finish prior to attempting any repairs on damage. By removing the finish, you can treat the raw wood itself and then re-do the finish as you wish.

2 methods to tackle black spots caused by Iron Oxide on wood.

Taking the veneer with black spots on it, I tested out two products to test which removed the spots easier.  Remember these are black spots caused by metal and water reacting on wood together. 

Oxalic Acid

Oxalic Acid tools needed for mixing a batch

One of the most important products any refinisher can have in their arsenal is Oxalic Acid. You can use it to remove any organic stains in wood and it can even lighten wood in general. 

Oxalic acid can be purchased in bulk, in its pure form or it is a key ingredient in Bar Keepers Friend.  I personal rather work with the raw product and mix up my own batch of oxalic acid when required.

Applying Oxalic Acid

Using a paint brush, I brush on my mixed oxalic acid and watched as the black spots caused by metal and water, almost immediately disappeared. For tough stains you may need to repeat multiple applications of oxalic acid.  

After each application allow the mixture to dry on the wood. 

Once you feel that the spot is sufficiently reduced or gone you can use plain water to wash away the oxalic acid powder that may remain on the surface after your mixture has dried. 

close up of black spots on wood
Black spots removed on wood

Bottom section; close up of black spots prior to oxalic acid being added.

Bottom section; close up of oxalic acid dried, leaving a white powder

Important to consider when choosing Oxalic Acid

a Shelf in a cabinet with visible stains

Oxalic acid lightens wood and can remove patina built up over the years.  When treating a specific area the black spot may be removed but the wood in the area will often no longer match the wood on the rest of the surface.  You will need to even out the tone across the whole piece by using oxalic acid across the whole surface. 

Stains removed with oxalic acid in cabinet. The wood is visibly lighter

Black spots on wood (L) and after two applications of oxalic acid (R) you can see the wood is lighter but the spots are removed.

Read for Full Details on how to work with Oxalic Acid

While this acid is a natural occurring product in plants and is simple to use, it is important to take safety precautions when working with the powder in its raw form. Please read this article before starting any project with oxalic acid in order to use it correctly,

Tannin Remover from Rubio Monocoat

Tannin remover from rubio monocoat

Black spots which are caused by metal and water together, is actually a reaction of iron oxide which has reacted with the tannins in the wood.  Therefore, I tried the tannin remover from Rubio Monocoat with surprising results. 

You simply spray on the tannin remover over the affected area and watch as the spots disappeared.  

Treating black spots on wood, with rubio monocoat tannin remover

Top Section after being treated with Tannin Remover from Rubio Monocoat

Benefit of the Tannin Remover from Rubio Monocoat

While Oxalic Acid can be used for multiple types of stains, I rather liked the tannin remover from Rubio Monocoat.  Specifically due to the fact that there was no additional clean up necessary as with the oxalic acid. Oxalic acid can leave a powder residue which needs to be tidied up.  Furthermore, Oxalic acid can affect the general wood tone and lighten up the area it is applied on.  I did not see the same affect with the tannin remover.  If you wish to avoid any tone difference on the wood, you may want to try the tannin remover from rubio monocoat first. 

Black spots removed, comparison to area with spots

Top Section treated with Tannin Remover from Rubio Monocoat.  Bottem Section with Oxalic Acid.  Middle line is untreated where iron oxide spots remain. 

Treating Other types of Damage resulting in black spots on wood

As I come across more types of damage resulting in black spots I will share my restoration processes in more details.  

Water Damage causing black spots

Are your black spots caused by water damage only?

Think a ring left by a plant pot. 

This damage is best treated with oxalic acid. 

You will need to remove the finish to treat the spot.  Oxalic Acid will remove stains caused by organic matter, including water damage.  It will, however, lighten the wood which could leave the spot looking rather different from the wood surrounding it.  You may wish to treat then the remaining wood to even out the colour difference or using pigments try and colour match the areas to how the finish will affect the wood.

What about mould and mildew on wood?

Firstly, if you believe you are working with mould or mildew, work in a well-ventilated area and ensure you take safety precautions.  Think gloves and respirator!

Ensure that the wood is first fully dried, or any actions you take could just result in the mould developing further. 

I recommend first starting with a water and vinegar mixture of a ratio 1 to 1.  Wipe the mixture on and leave it for a few minutes to allow the vinegar to kill the mould before wiping it off with a clean cloth. 

If you find that the vinegar and water mix isn’t working, you can opt for a bleach and water mixture of a ratio 1 part bleach to 4 parts water.  Please note that bleach can discolour wood.

Both the vinegar and bleach method can ruin any finish previously on the wood.  I would treat the mould first, then go in and remove the finish and treat the wood again if necessary.  Before sanding or refinishing, ensure all mould is removed and always use your respirator!

What about Burn Marks on wood?

Burns are harder to treat as they are damage to the wood or veneer itself. 

You need to remove all charcoal or burnt edges and sand lightly, until you have removed the full burn.  

Veneer could be repaired by simply cutting out the damage and repairing the spot with new veneer.  

Solid wood is slightly harder to repair.  You could fill the gap left by the burn with wood epoxy and then refinish in a method to mask the repair.

Frequently Asked Questions

Yes!  If you know what caused the black spot you can take precautions to avoid it occurring again. 

Avoid placing metal objects on bare wood, especially in damp conditions. Use coasters and protectors as a preventive measure.

Ensure plant pots are sitting on a dish or coaster and isn’t placed directly on the wood surface itself. 

And finally, do not place candles directly on a wooden surface and always use an ashtray to avoid causing burns on the surface. 

Generally, yes, but always test on a small, inconspicuous area first. Some woods may react differently.

It can vary, but you usually see results within a few hours. Be patient and follow safety guidelines!

You may not be required to refinish the entire piece, but the surface where the spot is, I would recommend removing the finish fully, so that you can treat the raw wood. 

Oxalic Acid and other products could ruin the finish further, therefore by removing the finish you avoid this additional issue.  

Once the spot is removed, you can refinish the surface to match the rest of the piece. 

Final thoughts

Black spots on your wooden furniture can be shocking and leave you thinking it is not salvageable.  Yet, the one thing I have learned about wood, is that almost everything is fixable.  Including water damage and unsightly marks.  

By determining what type of mark you are dealing with, you can easily tackle the issue with the right products.  

As I come across more examples of black spots on furniture, I will share how I restore the wood back to it’s former glory.  If you have examples you would love to share, reach out in the comment section or via email. 

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