Why has primer pulled paint off my wall?

[Snickersnack1]

I’ve been painting my home after recently moving in. Not sure what sort of paint used previously, but it’s extremely thin and I assume was whatever the builders used when it was built (newbuild, I am second owner). Anyway I couldn’t clean the existing paint without literally washing it off, so I have prepared the walls for painting using Zinsser Bullseye 123 Plus Primer.

No problems in the majority of the room, did it yesterday and it looks great. Today I prepare the final wall by the radiator and this has happened. I’m not sure why, or what to do about it.

Any thoughts about why this might have happened?

I did spot-test the paint I’m planning on using on this wall this morning. I did leave it until dry before painting over it with the Zinsser, but could that be the reason? It’s not flaking anywhere else, just this spot behind the radiator (which is not switched on).

Thanks

Behind a radiator you’d probably be better off with the zinsser BIN one which is better for areas exposed to heat.
did you lightly sand the walls before painting with zinsser?

Hi, no I didn’t sand the walls beforehand.

So if I sand away the flaking paint now, apply the BIN primer to the exposed plaster and then repaint over the top, does that sound like the right course of action?

Thanks for helping!

You should lightly sand walls or any surface really before applying primer so it has something to stick to. And then sand before each subsequent coat.
Give the whole area a really good sand to remove all the flaky bits then zinsser it, sand again then second coat of zinsser and then sand it lightly before applying your final coat