New exterior paint flaking off

[Lobster]

I know it's the wrong time of year for exterior painting, but needs must...

I've recently painted some masonry (lintel/window cill) using an exterior emulsion. Previous surface was pretty sound; gloss paint (probably oil-based I think). I cleaned the surface first and and sanded it very slightly to key it, and it was totally dry and also frost-free when I painted it.

However, within a couple of weeks, it's a total dog's breakfast; the new paint has cracked and flaked, and I can peel it off with my fingers.

So, once I've scrubbed the scabby paint off, how to sort it? Is the problem likely to be because the underlying paint was (I think?) oil-based? And/or not keyed enough? I want a quick'n easy solution ideally (house up for sale shortly!) - what would be best? Is there an intermediate paint or primer that would make the emulsion stick? Or should I attack the old paint with a wire brush to key it better?

Thanks David

[Pachiderm]

Not really a reply to your initial problem, David, but don't use a wire brush to prepare a surface for painting. It leaves a metallic film embedded into the underlying paint, which will make things much, much worse: I once used it ahead of eggshell-gloss, and the new pain just wouldn't 'stick' -- like painting on Teflon. I had to strip right back to bare wood ...

[Lobster]

Wot no answers?? (apart from what *not* to do!)

Surely this c> I know it's the wrong time of year for exterior painting, but needs must... >

[The Natural Philosopher]

yeah...I have this is certain areas.. over render.

I THINK the problem is a combined problem

The areas are exposed, subject to water splash, and MANILY on the north side of the house, in deep shade.

My CONJECTURE is that water is spalshing onto the render, through the breathable paint, and is freezing, and lifting the paint off.

What I intend to do is wait for summer, let it all dry our, strip any loose paint, paint with a water sealent top prevent more moistiue getting in, and repaint over that.

I don't want to entirely waterproof the render, as it needs to breathe a bit to clear any moistrure that does get in.

I'll let you know how it works next winter.. :-)

[Stuart Noble]

Moisture that gets in because it isn't waterproof you mean?

[The Natural Philosopher]

Not exactly, no. Moisture that gets in because its actcually got an airgap behind it before its hits the marine ply skin of the house. And a breatheable membrane so that any damp in the studowork structure can breathe outwards as well.