Flaking and blistering masonry paint.

Doesn't matter how good the stuff is, you still need to have a decent substrate, and gloss paintwork isn't it.

Presumably the cills are actually masonry ones?

I had a very similar problem myself recently - although I sanded down the old gloss paint to key the surface, it didn't prevent the new masonry paint flaking straight off. So I scraped and scrubbed it alll off and started again; next time I painted over the old gloss with an external grade, oil-based undercoat paint first. Then painted that with the masonry paint later - seemed to work fine.

David

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Yes, of course it affected it! It's intended to be painted onto masonry, not gloss paint.

Masonry is textured and absorbant and rough and dimply. Even my glosswork is smoother than that.

It's probably because the sills are made of concrete (or stone) and are damp to some slight extent. The moisture in the material will blister the paint as it tries to escape. You might do better to strip the sills entirely, and re-do with a "breathable" paint (Sandtex ultra smooth, for instance).

ok guys, seens like i need to remove the gloss paint first. How do i go about this... will nitromors paint stripper do?

You could use that, or a blowlamp and a steel scraper.

If it's onto masonry, it's likely to be very hard to shift via any means (surface being textured/porous) which is why I went for the method I did.

David

Another good trick is to use a watershield type coating before painting.

Yup. Pretty much., And a wire brush, and a pressure washer if you can beg/borrow/steal/buy one.

I think my best bet will be to 1)use a wite brush to remove as much of the flaking paint as possible.

Only one question. Where do i get this "external grade, oil-based undercoat paint" from and does it have a more common name. Isn't exterior gloss external grade, oil based paint. How would it being undercoat, rather than topcoat affect it?

Try a trade decorator centre, Crown (owners of Sandtex) or Dulux perhaps. (You could also ring the tech advice line - the number's on your Sandtex can.)

It's just standard "undercoat" i.e. not "quick drying". Available in all the sheds. Often the instruction to wash brushes in white spirit is all that indicates it's oil based. If the sills are going to be white, I wouldn't bother with masonry paint