Black stains on wood

[C08]

I have some rather nice garage doors I made with pine planks and playing with a router. They have managed very well for 12 years with a yearly dosing of Dekolje soaking oil but without the finish oil coat which is where I may have erred. Over the last year black(mould?) streaks have developed along some planks where the DO coating has weathered. Sanding down does not work as the black penetrates beyond the surface. Is there chemical I can apply or any other method to remove the black staining before re- Deksolejing?
Apology if this is thought to be in the wrong forum but this is where the expertise with wood is.

 

[Poignard]

Oxalic acid

 

[Humblebee]

Oxalic acid

Seconded. Widely available on the Internet, read safety guidance notes before using.

 

[C08]

Thanks I will try Oxalic acid and report back on success..or not.

 

[C08]

Thanks for advice-unfortunately Oxalic Acid did not work at all used in various concentrations/mixes. I will just have to woodstain them!

 

[Humblebee]

Thanks for advice-unfortunately Oxalic Acid did not work at all used in various concentrations/mixes. I will just have to woodstain them!

Just an idea, before you start staining them,
I had a similar experience this winter when tidying up the tiller. I thought I had removed what remained of the old varnish but then the results I got with Oxalic Acid were patchy and unsatisfactory. It seems I hadn't removed the old varnish as thoroughly as I should have done and bits of varnish remained, preventing the Oxalic Acid reaching the wood and doing it's job.

 

[AntarcticPilot]

Just an idea, before you start staining them,
I had a similar experience this winter when tidying up the tiller. I thought I had removed what remained of the old varnish but then the results I got with Oxalic Acid were patchy and unsatisfactory. It seems I hadn't removed the old varnish as thoroughly as I should have done and bits of varnish remained, preventing the Oxalic Acid reaching the wood and doing it's job.

I've had the same experience with interior woodwork. If you don't get all the old varnish off, oxalic acid doesn't penetrate. Also, you need to leave it on for a reasonable period - hours, not minutes.

 

[C08]

MMM perhaps because the doors were treated each year with Deks Olje and they were fine until last year when the doors were not treated and the black appeared. It may well be that enough of the DO remains to stop the Oxalic Acid doing its magic but not enough to protect the wood. Sods Law in operation?

 

[Hydrozoan]

For reasons I explain here, if oxalic acid does not work (and not simply because of a residual barrier to it), an oxidising, chlorine-based bleach such as diluted household bleach - applied with appropriate precautions - would be my next step.
How to remove black mold from wood