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

Surely this conundrum isn't above everyone here? :-)

Lobster wrote:
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?