How to remove black mold from wood

[moomba]

Oxalic Acid, the active ingredient in Y10 and other cleaners, should do the trick. Cheap and widely available on the Internet. Handle with care though and look into safety instructions before use.

+1 for oxalic acid in its pure form diluted as recommended , took out all black stains in my teak all inside then sanded and varnished , need to be carefully for an even layup
this is not a man made chemical but natural product so should be ok on your wood,

 

[greeny]

Is that oak in the laminate? If so it's not unusual for oak to stain in this way, and it probably will do again especially if the varnish protection is compromised. I have the same problem with my front door in UK. Oxalic acid works to remove it then a good varnish to protect it. If you're stripping the tiller don't use wirewool or anything that will leave ferrous particles behind in the wood or it will invariably return sooner rather than later. I found this out the hard way on my oak front door and frame.

 

[Jonny A]

The staining is in both woods in the laminate. I'll give the oxalic acid a go then.

 

[Hydrozoan]

I had some small areas of mould staining on my laminated tiller where the varnish had failed. I used a cheap artist’s paintbrush to apply thick household bleach to the affected areas (usual precautions) and rinsed it off thoroughly after leaving for some hours. Several applications were needed in some cases and it may be impossible to remove the last traces if the stain is deeply ingrained. (Maintaining the varnish is much simpler now I have a tiller cover.)

 

[thinwater]

This is what it looks like:
View attachment 75051

It's too deep in the grain to possibly sand out. I don't think it's rot though, the wood itself looks fine, it's just discoloured. And I'd really like to get it removed before I re-varnish.

I feel bad enough putting bleach onto the wood as I'm worried about leaching out the oils. And I don't want to use any sort of patio cleaner as I don't know what all the ingredients are - I don't want some sort of residue buggering up the varnish.

First, don't worry about the residues or buggering the varnish bonding. I've been doing long terns testing with samples buried in compost, and at year 3, no problems. I have also used this chemistry for longer on the boat (cockpit grating). Finally, Halberg Rassy recommends this chemistry (Boracol, which contains both borax and Patio Magic).

The problem with bleach is that it doesn't penetrate. It reacts on the surface and is neutralized before it goes in more than a few mils. This is true of bleach in all sanitizing applications; the surface must be clean first.

Oxalic acid is AKA wood bleach. This is not a new application. It has been used by wood workers for generations and is used in some traditional bleaching oils.

The chemicals mentioned will not interfere with varnish, but you will need to allow a LOT of drying time for Boracol; it contains glycol and is slow to dry.

 

[Jonny A]

Thanks for the detail Thinwater. I already have some oxalic acid on order and will see how that goes.

 

[alan_d]

+1 for oxalic acid ...[snip]... this is not a man made chemical but natural product so should be ok on your wood,

I'm not sure how useful or meaningful the chemical/natural distinction is, here or anywhere else. Although oxalic acid can be extracted from plants, the stuff you can buy is more likely to have been synthesised. The notion that natural products are inherently safer than synthesised ones is not one that bears close examination. Belladonna and foxglove are natural, but you wouldn't want them in your salad, and the fact that botulinum toxin is produced naturally doesn't stop it being extremely dangerous.

 

[smithy]

Oxalic acid works well for this but the surface has to be reasonably warm. It requires patience and several applications on more stubborn stains. Neutralise the surface with some baking soda afterwards. The grain will raise and will require very light sanding, be careful on veneer. It will bleach the surrounding wood and this will usually require stain to get the original colour back.
Its a slow process best practiced somewhere that doesn't show.

 

[jwilson]

As most others have said it's oxalic acid that will lighten the stain. Use at as concentrated solution as you can get mixed, use somewhere warm, ideally 20C or more, wash off afterwards, then varnish. Won't work well in cold conditions - not sure why. May not get wood back quite to original colour but will look much better.

Use the rest of the acid to remove rust stains or brown waterline stains from white GRP - on a warm day.

 

[moomba]

I'm not sure how useful or meaningful the chemical/natural distinction is, here or anywhere else. Although oxalic acid can be extracted from plants, the stuff you can buy is more likely to have been synthesised. The notion that natural products are inherently safer than synthesised ones is not one that bears close examination. Belladonna and foxglove are natural, but you wouldn't want them in your salad, and the fact that botulinum toxin is produced naturally doesn't stop it being extremely dangerous.

the meaning behind it was that it is a less harsh chemical that can be sold and transported without any restrictions and n a natural surface like wood the acid can work less aggressively ,
Oxalic acid is an organic compound with the formula C₂H₂O₄
No you would not want Foxglove in your salad nor would you like to touch it if you have a heart condition :encouragement:

 

[Mistroma]

the meaning behind it was that it is a less harsh chemical that can be sold and transported without any restrictions and n a natural surface like wood the acid can work less aggressively ,
Oxalic acid is an organic compound with the formula C₂H₂O₄
No you would not want Foxglove in your salad nor would you like to touch it if you have a heart condition :encouragement:

this is not a man made chemical but natural product so should be ok on your wood,

I must admit that I also thought you meant it was safer because it was natural rather than man made. Organic in this context does not infer anything about the chemical being less harmful. The formula doesn't really give most people much help either. The spatial arrangement can also have a huge impact on the effect of chemicals with the same formula, though possibly not significantly in this case.

Availability isn't that relevant either. I'd consider Caustic Soda to be pretty aggressive even though you can buy as much as you like very easily. Alan_D's clarification did seem to be worthwhile.

 

[moomba]

I must admit that I also thought you meant it was safer because it was natural rather than man made. Organic in this context does not infer anything about the chemical being less harmful. The formula doesn't really give most people much help either. The spatial arrangement can also have a huge impact on the effect of chemicals with the same formula, though not significantly in this case.

Availability isn't that relevant either. I'd consider Caustic Soda to be pretty aggressive even though you can buy as much as you like very easily. Alan_D's clarification did seem to be worthwhile.

Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound
The reason why oxalic acid is used on wood and stain removal of dirt on a fiberglass hull is that it is not powerful,reflecting its classification as the simplest dicarboxylic acid. The term acid normally envisages a corrosive liquid , were Oxcalic acid is organic in nature Oxalic acid is an organic compound found in many plants.These include leafy greens, vegetables, fruits, cocoa, nuts and seeds.
In plants, it’s usually bound to minerals, forming oxalate. The terms “oxalic acid” and “oxalate” are used interchangeably in nutrition science.
If used correctly it is a good cleaner and degrease and will whiten marked wood, it is also safe to handle and use , (dont drink)
For the restructuring of a chemical compound another atom from another source must be added and then the formula must balance or the compound in question will not be stable so the organic compound of Oxalic acid cannot be changed or altered r you will get a different compund

 

[Mistroma]

Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound
The reason why oxalic acid is used on wood and stain removal of dirt on a fiberglass hull is that it is not powerful,reflecting its classification as the simplest dicarboxylic acid. The term acid normally envisages a corrosive liquid , were Oxcalic acid is organic in nature Oxalic acid is an organic compound found in many plants.These include leafy greens, vegetables, fruits, cocoa, nuts and seeds.
In plants, it’s usually bound to minerals, forming oxalate. The terms “oxalic acid” and “oxalate” are used interchangeably in nutrition science.
If used correctly it is a good cleaner and degrease and will whiten marked wood, it is also safe to handle and use , (dont drink)

Pity I'd never been a research chemist or I'd have known that caustic soda was an inorganic compound. Oh, wait a minute, I was but it was a long time ago. I was simply using it as an example as you inferred that dangerous chemicals wouldn't be sold or transported without restrictions.

For the restructuring of a chemical compound another atom from another source must be added and then the formula must balance or the compound in question will not be stable so the organic compound of Oxalic acid cannot be changed or altered r you will get a different compund

Nope, check out stereochemistry, that's why I mentioned spatial arrangement. Different enantiomers can have vastly different effects. Hence problems cause by Thalidomide before people realised that it could be a problem. Pretty common for one enantiomer to cause side-effects and other to give the desired effect but the formula doesn't give that information.

 

[moomba]

Pity I'd never been a research chemist or I'd have known that caustic soda was an inorganic compound. Oh, wait a minute, I was but it was a long time ago. I was simply using it as an example as you inferred that dangerous chemicals wouldn't be sold or transported without restrictions.



Nope, check out stereochemistry, that's why I mentioned spatial arrangement. Different enantiomers can have vastly different effects. Hence problems cause by Thalidomide before people realised that it could be a problem. Pretty common for one enantiomer to cause side-effects and other to give the desired effect but the formula doesn't give that information.

We are talking simply Organic Chemistry here and cleaning wood you used an inorganic material in comparison to a organic , completely different ,
we are talking a well and proving method of wood cleaning for centuries which is Oxalic acid ,
What would I know just studied Organic Chemistry at Uni,
But this is a boat forum so lets keep it at that

 

[Mistroma]

We are talking simply Organic Chemistry here and cleaning wood you used an inorganic material in comparison to a organic , completely different ,
we are talking a well and proving method of wood cleaning for centuries which is Oxalic acid ,
What would I know just studied Organic Chemistry at Uni,
But this is a boat forum so lets keep it at that

No problem, I thought you were talking about hazardous chemicals in general. I was just amazed by your apparent lack of knowledge of stereochemistry. Pretty basic stuff but to be fair it might not be covered in initial chemistry courses.

But yes, not relevant here so worth forgetting about.

Back to Oxalic acid to try actually contributing something worthwhile. I've used it for about 40 years and of course it does work. I saw earlier comments about temperature and would like to confirm that is pretty important. I did once speak to someone who claimed it didn't work well but had only used it in winter. I don't think I have seen a graph indicating approx reaction rate vs. temperature for 1/3 century. My recollection of the shape was that you'd expect to wait months for much effect around zero C.
At 10C it's more like hours
At 20C, maybe minutes
30C, you probably see an effect within seconds.

Warm the hull with some hot water for localised stains and use a warm solution. I use a sponge mop to keep it at arms length and avoid it running down my arm. Works well, especially in Greece where lack of heat isn't usually a problem.

 

[moomba]

No problem, I thought you were talking about hazardous chemicals in general. I was just amazed by your apparent lack of knowledge of stereochemistry. Pretty basic stuff but to be fair it might not be covered in initial chemistry courses.

But yes, not relevant here so worth forgetting about.

Back to Oxalic acid to try actually contributing something worthwhile. I've used it for about 40 years and of course it does work. I saw earlier comments about temperature and would like to confirm that is pretty important. I did once speak to someone who claimed it didn't work well but had only used it in winter. I don't think I have seen a graph indicating approx reaction rate vs. temperature for 1/3 century. My recollection of the shape was that you'd expect to wait months for much effect around zero C.
At 10C it's more like hours
At 20C, maybe minutes
30C, you probably see an effect within seconds.

Warm the hull with some hot water for localised stains and use a warm solution. I use a sponge mop to keep it at arms length and avoid it running down my arm. Works well, especially in Greece where lack of heat isn't usually a problem.

Like I said simple , the OP asked for removal of a mark , Oxalic acid is as proven organic acid , therefore not a harsh substance for his wood.
It can be delivered without being a hazardous substance, it can be used without fear of burning to the skin, it is gentle on the wood ,
this is why I wrote what I wrote, simply chemistry :encouragement:

 

[pvb]

Like I said simple , the OP asked for removal of a mark , Oxalic acid is as proven organic acid , therefore not a harsh substance for his wood.
It can be delivered without being a hazardous substance, it can be used without fear of burning to the skin, it is gentle on the wood ,
this is why I wrote what I wrote, simply chemistry :encouragement:

Oxalic acid is very irritating to the skin, and can be harmful if absorbed through the skin. That's why products containing oxalic acid always come with prominent warnings about using it safely.

 

[moomba]

Oxalic acid is very irritating to the skin, and can be harmful if absorbed through the skin. That's why products containing oxalic acid always come with prominent warnings about using it safely.

Unless rubbing your hands in in and putting your hands into it your are pretty safe, but like any chemicals it is always advised to wear gloves , good to see my shadow is following me:encouragement:

 

[Mistroma]

Unless rubbing your hands in in and putting your hands into it your are pretty safe, but like any chemicals it is always advised to wear gloves , good to see my shadow is following me:encouragement:

I imagine PVB is worried about splashes on exposed skin. It can be a problem if people just think it's not a big deal. I have seen some horrible injuries from caustic and also fairly weak acids. Contractors would ignore a splash and only notice after a couple of hours. Certainly worth being careful to wash immediately with copious amounts of water if you do have some on your skin.

It won't be immediately fatal by any stretch but it does accumulate in the body so best to wash it off. Keep it away from eyes as well. Oxalic acid solution can cause irritation or burns but at least absorption through skin takes a little while. I imagine you'd need to swallow a small spoonful of crystals to get a lethal dose. At least that's highly unlikely to happen accidentally.

 

[Hydrozoan]

Whilst oxalic acid is used to bleach wood, it is considered in conservation work to be (as a chemically reducing bleach) one of the weaker bleaches with respect to stains and dyes (see page 6 here https://www.diva-portal.org/smash/get/diva2:533607/FULLTEXT01.pdf) – and as the work in that reference shows, also a weaker bleach with respect to the natural pigment compounds in wood. Chemically oxidising wood bleaches are generally stronger.

Oxalic acid is I believe very effective for dealing with stains from contact of iron with (for example) oak because it is a strong chelating agent, and so destroys (by preferential complexation) the iron complexes with tannins which cause the staining. (That mechanism means that it will be more effective in that role than weaker, non-chelating complexing agents such as citric or acetic acids.)

I have failed to find on the general web details of the chemical nature of the pigments in black mould. If they are generally similar to those naturally occurring pigments in timbers and those in artificial wood stains and dyes (rather than complexes with iron) oxidative bleaches might be more effective. I was not a specialist in organic chemistry, and perhaps Mistroma and VicS have more knowledge of the nature of the pigments in black mould?

In any event, if oxalic acid is not adequately effective, one could move progressively to an oxidising bleach – with the advantage that the oxalic acid is less likely to heavily bleach the wood itself.

Finally, I would reiterate the point made by others that appropriate precautions should be taken with any bleaching agent – the fact that one might be found in nature is no guarantee that it will be less harmful.