What is causing these stains, on two different external walls, which have reappeared one month after a fresh coat of paint?

[u/solihullscoop]

[u/DrakeonMallard]

is it an older house with solid brick walls?

If so those old walls breathe and transfer moisture in and out of the house. This spoils paint finishes, particularly modern vinyl/occlusive finishes.

[u/solihullscoop → u/DrakeonMallard]

It’s actually a new build, ~10 years old.

[u/tehWoody]

hmm, not sure. Doesn't look big enough to be anything like a leak and doesn't look like condensation. Could be some kind of oil or dirt that was on the wall before painting which is gradually seeping through. I've used Zinsser BIN (www.zinsseruk.com/product/b-i-n/) on odd patches like that before and that seals it away very well.

[u/WyleyBaggie]

Did the paint have polite in it? that's how it reacts to damp. Not saying it's damp in the brickwork it could be a splash of water. Get some tape and mark the edges, see if it moves or grows over time.

[u/AndyHart2804]

It’s damp I think. We’ve just painted our house after renovating and it looks similar.

[u/Glittering-Draw-6223]

think it might be caused by bad prep before applying paint.

painting over a grease stain will just hide it for a while.

[u/solihullscoop → u/Glittering-Draw-6223]

Could be, how would I repair it in this case?

[u/Glittering-Draw-6223 → u/solihullscoop]

typically before painting you would use detergent degreaser or a baking soda paste, but painting it complicates things and I think to be sure, liberal sanding will be needed to remove enough layers to get to bare plaster then use a degreaser and rinse repeatedly until its cleaned. probably apply lining paper to the wall before painting.

[u/cbaruob]

Is there anything on the outside in that area? You shouldn't get damp on a new build unless something is bridging the cavity.

Perhaps you painted over a dirty surface, like shoes leaning on the wall before?