Mould on wood

[Slow_boat]

I have a 20yr old Sadler which I don't think has had the cabin varnish touched ever. I can easily feel the grain of the wood beneath the varnish and there is black mould under the varnish in some places. I intend to strip with Nitromores and start again with matt varnish but how do I get rid of the mould? Is it likely to spread if try to sand it out?

 

[tillergirl]

Black staining under varnish is usually a sign of dampness. If I see it on TG and in the early days of my ownership before I got maintenance under control I did, it is a worrying sign of damp coming through from the other side of the timber. I doubt it is mould; I think that needs air. In that respect it is unlikely to spread but the source of the dampness needs to be tackled.

 

[longjohnsilver]

Try oxalic acid, should get rid of any mould discolouration.

 

[Twister_Ken]

Echo the oxalic recommendation, but it might need several applications if it is deep.

 

[James_Calvert]

I've had similar problems on my 85 Sadler. It's the flat areas - which are teak faced ply - which get affected in the grain of the veneer. Agree you need to find out why the wood is getting damp and stop it.

As to a cure, I've not tried oxalic acid, but have found it possible to manually sand out the black bits with wet and dry used wet. They can be quite deep so sanding them out tends to emphasise the original graininess of the finish.

I've not gone through the veneer yet ...

Please let me know how you get on.

 

[tillergirl]

I'd go for repeated goes with oxalic first. Sanding a veneer is pretty risky if you want to keep it. But if it is ply, I would suggest trying to find the cause of the dampness is even more important.

 

[Juggler7823]

Oxalic acid does work but its sloooooooowwwww. Try the mould removal kit from Fibbes at Penarth. Try Googling for the web address

 

[LittleShip]

On Teak it is normally black/blue ish! it is mould and needs removing before varnish or it will continue to spread.

remove the varnish, clean with oxyalic acid, (if very bad you can use bleach to remove and kill, but be careful not to over bleach) then completly dry, sand (lightly) and then varnish.

If it is a small area you may have to clean he whole peice as it will lighten after treatment.

Tom

 

[doug748]

Funny, there was a very recent post on this but I could not find it.
I am not convinced that the science of this problem is properly understood. These stains seem to always originate in the open grain of the wood, almost never in solid timber. They are often on the front of a panel, in the light ,and absent at the back where you might expect mould growth to be worst.
It is my suspicion that it is nothing to do with dampness per se but is a chemical decomposition of the old varnish prompted by the marine atmosphere. If this sounds like the same thing in different words then perhaps it is, but I think there is a subtle difference.
Anyway treat as suggested. In my view don't look for any excessive damp problem I doubt you will find one; and do not be obsessive about removing every last trace of black, your treatment should knock it back for another 20 years.
Perhaps there is a Phd in this for a Chemical Biologist.