Black stains/marks on wood left outside in rain

[JohnPW]

Hello all,

I've got a rubber wood table top that I found left on the pavement, propped up against a wall. I think it's been rained on at least twice. The unfinished parts where water has collected have got blacks marks. I wiped it with a damp cloth and the cloth got black, the cloth washes clean easily. But the wood still has the black marks, although a bit lighter now.

I presume it's caused by mould or fungus.

Is it OK to leave it there, I'm going to use the wood for a work bench top so appearance doesn't matter, but I think I'll cut away the affected wood. But if I were to keep it, will the mould or fungus spread in the wood and get worse, or even go on to affect other pieces of wood, like woodworm?

Pic of wood after wiped:

 

[celston]

Once dried, the mould will stop but as you already suspect the staining will remain. There should be no ongoing effects as long as it's kept suitably dry.

 

[Sgian Dubh]

It might be mould because rubber wood is rather susceptible to mould attack. But it's also slightly possible the black marks are the result of water reacting with tannin in the wood. I say slightly possible because I don't think the tannin content of rubber wood is high.

Anyway, assuming the marks are mould, the easiest way to stop its growth is simply to reduce the wood's MC to 20% or less. Once you've got the wood down to this level of MC, as long as you keep the wood dry you won't have further mould problems. Basically, this means placing it in a reasonably dry shed to hold the MC below 20%. Keep it in a dry house and its MC will sink to somewhere around 8 - 12 % MC. Kept outside, the MC will rise above this level at some point, and mould growth is likely to recur.

Getting rid of the existing marks might be possible through some planing, scraping and sanding, but the success of this will depend upon how deep the staining goes. Alternatively, an A/B bleach might do the job, but the resultant colour will be an off-white, sort of yellow'ish, which may or not be a problem depending upon the desired end use and appearance preference. Slainte.

 

[dickm]

Oxalic acid? (to remove the staining)

 

[Myfordman]

+1 for oxalic acid.

MM