My pal has some garden furniture that was originally finished in wood stain. Some time ago he varnished it. The varnish has cracked in places and there are widespread black marks under the varnish. Has anyone else seen such a thing?
Bill
Probably a dark green algae or a combo of a red one and a green one.
Moral of story is to use wood stain or oil on garden furniture. It is a very demanding application. Best stuff I found was Dutch Sikkens (sp?) from Dulux decorator centres - it was a coloured base coat and clear UV formulation.
Yacht varnish will hold up for a while but even that fails eventually. I have just had to redo our house sign for that reason.
It's probably where rain has penetrated the varnish and caused the staining. Probably not going to be removeable without a lot of effort/sanding down. Best to use woodstain rather than varnish.
One of our 7 original pubs (now only 2 left) was converted into 4 flats with nice light oak front doors facing south-east about 15 years ago.
The bottom 8 inches of these doors have now discoloured badly with lots of black spots. I suspect the decorator who finished them originally didn't bother to read that bit of the manufacturers instructions which said "All exposed sides including top and bottom must be sealed immediately before final fitting."
There are still 'chippies' out there who (mis)believe that you should leave the top and bottom of an external door as bare wood "so that it can breathe".
Yeah, varnish sometimes does that.
Sound like water gets in under the cracks and causes the stain... algae, mold, or some sort of chemical reaction -- that last would be common in oak.
Sand it all off and re-stain, sand off the worst bits and re-varnish, maybe using a colored varnish to equalize the old and new bits. Or possibly bleach will take out the black marks, maybe after taking the cracked bits of varnish off so the bleach can reach the stain -- sandpaper or cabinet scraper.
Bleach could be standard chlorine bleach, or lye/hydrogen peroxide, or oxalic acid. All have their dangers -- read up before applying:-)
All of the bleaches can cause the stains to recede beautifully, leaving the rest of the wood untouched, and the whole thing looking crap. This necessitates sanding *everything*, and then the bleach doesn't work as well on the previously unbleached bits, leaving the whole thing looking still crap, at which point you stain the thing as dark as it will go -- which you could have done right off, i.e. slap dark varnish right over the cracks, and run off leaving a "good enough" job...
Thomas Prufer
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