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Cleaning Tips Porches & Decks Exteriors

Black spots on new outdoor treated wood?

8 months ago

Hi all,
Just build an outside gazebo and the slate batons we used for the roof shaded sections is quite rough (price choice) and had quite a few black marks on. We presumed it was OK as all the batons had this on and it's from a popular local wood yard.
They have been stored outside but my mother who we built it for, now doesn't like the black marks as they have got worse. The roof is covered but lots of air so I don't imagine it's mould.
We thought perhaps oxidation with iron and the preservative?
But the little black spots are exactly where the green wood preservative is so it's those parts that are turning black.
Trialing oxalic acid at the moment on the worse ones but I cant possible clean every single baton as there are hundreds of them!
Is this normal? Will it disappear eventually? Argh.
Ps a bit of it came off with bleach when wiped over but not all of it - hence why we don't think it's mould.
Thank you

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Comments (2)

Sonia
8 months ago
It looks like a black algae that can grow on wet wood. Is the location in a shaded area? I have a patch on my raised pond where the water from the fountain sometimes hits the wood. I have used Oxalic acid on my oak worktop where it sometimes gets wet and goes black. It really does bleach the wood. If you do use it make sure you wear a good quality mask as the crystals can get into the lungs and cause problems. I read spraying with vinegar may help?
Tani H-S
Original Author
8 months ago
Hi Sonia! Thanks for replying..

Most of it was like that when we bought it from the wood yard but it was during that rainy month so it had probably been sat out for a while.

I tried the oxalic acid yesterday but although some darker patches around the knots came up lighter, it didn't really draw out/get rid of the little dark patches. Anything that came off seemed to be on the surface ie so a mould /algae layer but I think the stuff left was the excess tanaliser which was that light green but had turned black.

No idea where its origin is but the green powder looked almost like verdigris colour so perhaps something has reacted with the it and turned it black? On one section, half will be black spotted and half will be the green colour.

I also tried a mould and mildew remover for outside wood but that didn't work either 🤔

Not tried vinegar but thinking it's not a mould or algae if it's in the wood?? It's been treated prior to purchasing so...
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