How to remove black mold from wood

[Mistroma]

Hydrozoan, so spot on with chelation.

We were tied up alongside a brand new big Oyster in Baiona several years ago. I was chatting to the only person on the boat, a very young guy who had just returned to the boat. He was waiting for the owners to pick up the boat and was expecting the sack. He'd been on the crew dropping off the boat and been left behind to watch her and keep her looking brand new. Unfortunately, he thought he had time for a few days away from the boat. He'd arranged for gas bottle refills and left 2 or 3 sitting on the cockpit floor for collection. The deck had just been washed and it also rained the following day after the new bottles had been dropped off. The cockpit looked like an advert for the Olympics when he returned. Overlapping rust rings everywhere.

I had a quick look, explained that he was lucky they were rust stains and introduced him to Oxalic acid. He could not believe it when the deck returned to a new looking unmarked state. I said he was lucky that it was rust and that the deck was still brand new as it wouldn't change colour much, just have the rust removed.

I gave him a safety briefing, notes on what it could and could not do. before leaving him some Oxalic acid crystals. He swore that he'd keep a small stash when crewing on any boats in future.

 

[Hydrozoan]

I always try to think carefully about what is to be bleached or cleaned away – which is why I’d like to know more about the nature of the pigment(s) in black mould. (I have also tried Google scholar, somewhat perfunctorily, but without success. I do know that black mould can be very refractory, when impregnated into textiles for example.)

That approach does not, however, guarantee success. When someone unwittingly left a wire-framed wreath on my FiL’s gravestone – a slab of rare purple slate – I tried (after some years) to remove the severe rust stain with a proprietary iron stain remover (likely oxalic-based – I do not recall) only to find that it also much weakened the slate colour – which is probably also due to iron in the slate. Fortunately, I had forewarned my MiL that success was not at all certain, and the result was at least a significant improvement.

 

[Jonny A]

Thanks everyone for what has evolved into quite a useful course in organic chemistry.

 

[Red Panda]

Did you manage to remove the staining?
I ask as I'm trying to do the same on my tiller, which looks pretty much like the "before" picture you posted. I've been using oxalic acid but with limited success so far.

 

[Jonny A]

Hi Red Panda, I managed to get 90% of it off by repeated applications of 50-50 bleach and water, after some heavy sanding. I've now got it varnished and the result isn't immaculate but it's good enough, and I'm pretty fussy.

 

[Red Panda]

Hi Red Panda, I managed to get 90% of it off by repeated applications of 50-50 bleach and water, after some heavy sanding. I've now got it varnished and the result isn't immaculate but it's good enough, and I'm pretty fussy.

Thanks - that's encouragement enough for me to continue the process.

 

[[159032]]

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

So, did you use oxalic acid? I would be surprised if it hadn't worked well. I've done stained oak dinghy transoms with it

 

[greeny]

did my oak front door with oxalic acid 2 weeks ago. No black spots now.

 

[Jonny A]

So, did you use oxalic acid? I would be surprised if it hadn't worked well. I've done stained oak dinghy transoms with it

No, in the end I just stuck with the diluted bleach and sanding as they seemed to work OK.