Black mildew in varnished wood

[Edwardo]

I seem to have patches of black mildew engrained into varnished marine ply in the cabin. How do I get rid of it as scouring has no effect.

thanks for any ideas

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[Freebee]

This appeared on a boat I had it turned out to be the appearance of rot underneath the varnish.

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[snowleopard]

not much chance of shifting it. you'll have to remove the varnish then dry out thoroughly and sand it. if the rot is well advanced it will go right through the top veneer so you're stuck with painting or putting up with it. chemicals will probably have more effect on the good wood than the black bits.

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[Edwardo]

Does this mean I am stuck with the marks or are they irradicable?

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[snowleopard]

mildew removing solutions contain bleach which will affect the colour of the wood. you're stuck with having to paint it or put up with the black marks (unless you go in for something radical like re-veneering or replacing the ply.

sorry to be the bearer of bad tidings but i've been there more than once.

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[ronniewood]

Thoroughly remove the varnish - not just by sanding but using a varnish remover.

Make up a wood bleach by getting some oxalic acid crystals and make up a paste which should be applied to the black marks. It's a bit hit or miss on the time to leave the paste in place while it bleaches the wood but we started at 2 hours and extended untill the bleaching had worked. Wash off any residue and let the wood dry. May take a few applications depending how bad the staining is.

When thoroughly dry, sand as usual and revarnish.

Found the best place to get the oxalic acid was a Company supplying chemicals to school/college labs.

Takes time but usually gives a good job.

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