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.
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.
Photo of water mark curtesy of Furniture Restoration Expert Marianne from Brakendonk.
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 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.
Photo of burn mark curtesy of Furniture Restoration Expert Marianne from Brakendonk.
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.
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.
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.
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
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.
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.
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.
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.
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
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.