Bloody stuff. South facing windows, a year or two, then off it comes. If I ever make more windows, I don't think I'll paint them, just use linseed oil. Slap it on, the wood sucks it up, recoat as needed without having to riss about with sanding, stripping, etc. Something like that, anyway. I'm experimenting with satin acrylic right now. I do not have high hopes.
Grr!
I have a "redwood" stain I use on fence and steps, and it seems to go over most materials. It even goes over wet pressure treated lumber (which surprised me). The previous stain was petroleum based, while the new stuff is supposed to be some sort of crossover material.
One thing it won't go over, is mold/mildew. You can tell by the way the liquid smears and doesn't wet the surface, that finish failure is guaranteed.
If there are trees overhanging one end of a house, that can lead to the surfaces being polluted.
There are plenty of cans of stuff at the paint store, unsuited to even piss in. But there are occasionally a good product or two, compromised by biofilm on the surface. If you notice "something funny" when wiping the stuff on with a brush, that is your warning you did not prepare the surface well enough.
I've avoided addressing the issue of "what removes biofilm", because I don't know. You would think "angle grinder" or "sand blaster" - I'm not convinced that's the right approach.
Paul
I had Sadolin Superdec put on my house's external woodwork a few years ago. It's supposed to last ten years, but on the south facing front it barely lasted five. That's when I got uPVC fascias and soffits installed.
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