New paint flaking on one bit of wall mystery

[djini]

I've been repairing my old study to turn into a new bedroom for DD(4). It was previously painted blue (by the people before us), then some shade of white (by us, 5 years ago) and now I'm putting on a coat of duplicate just walnut matt (left over from living room). House is from 1920s. All previous paint jobs have been fine and smooth on the walls. I haven't removed any of the previous paint before starting each new one because I'm a lazy corner-cutting arse.

Now, after the first new coat, one part of one wall (exterior) about the size of a ten point note has got paint bubbling and flaking where the bubble "pops".

I googled, and then applied a mist coat to the affected bit, gave it 48 hours and repainted. And it's done it again! If I get rid of just the peeled paint and paint over, the new coat peels around the edges of the bit I removed before, making NGOs the damaged area bigger. And so on. At this rate, the whole wall will be affected.

What am I doing wrong and how can I fix it? Help!

Damp patch? something greasy on the surface? Give it a wash with sugar soap maybe?

Hmm - sounds like there is something on the wall which is reacting with the fresh paint. How quickly did it bubble the first time?

As the paint was drying you could see it starting to bubble, then when it was dry it flaked.

I think there's something on the walls which is reacting with the paint. The fact that it's now only flaking around the edges of the original patch (did you sand to remove the flaky paint?) suggests your sanding has removed whatever it was. So, you can either sand a wider area and try again or you could a stain stop paint or primer to seal the area and re-paint.

How old is left over paint you are using? It's possible it has deteriorated or not been sufficiently mixed before using.

Ok, I've sanded around the area and put a stain stop coat on (just the affected bit) now. The paint I was using was a few years old but hadn't flaked anywhere else in the room so my money's on your theory about a grease patch or similar on that spot.

Bloody annoying, in any case!