In article ,
Harold Davis writes:
Hello,

I need to paint the concrete window sills and the window surrounds on my
roughcast kit bungalow near the Atlantic coast of the Isle of Lewis in
Scotland. They're painted at the moment but haven't been maintained for some
years. The sills I think have been done with some kind of matt external paint
and are crumbling but only very slightly and on the surface and not
sufficiently to require skimming.

Possibly masonry paint?

The surrounds have been done with something more glossy.

The guy in the paintshop recommended a 3:1 water:PVA coat after scraping, and
then some Dulux Weathershield. He didn't seem to know much about SBR or
alternatives to that brand and type of paint.

Sounds like a bad idea. PVA isn't waterproof - it's water soluable.
Even EVA (external PVA) isn't waterproof, except inside mortar mixes.
SBR is, but I don't know about longevity in highly exposed situations.
Dulux Weathershield doesn't last long on masonry (from personal
experience).

I don't have an answer, but I would have thought a masonry paint would
be most appropriate and hence longer lasting.

Should I use PVA or SBR or something else? I thought for a while that maybe
he's right at least on the PVA score because a decent paint will soak into the
concrete which SBR being waterproof would hinder. I'd be grateful for advice
from some of the knowledgeable people here.

And what should I do about the surrounds?

Third question... The walls are roughcast and look mostly okay for the time
being although in places they are weather-stained and I plan to paint them. I
was thinking one coat of SBR followed by two coats of masonry paint. Does that
sound okay? Or would it amount to putting a vapour barrier on the cold side of
insulation and thereby muck up the ventilation something rotten? I'd like to
use paint that doesn't need to be recoated in the next 20-30 years.

I think that's a bit of a challenge. For that sort of life, I
believe coloured renders are normally used.

--
Andrew Gabriel
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