Looks like paint runs but it isn't !

[saab9002]

Hi All,
I've recently finished redoing our bathroom, the whole lot,new suite,shower retiled etc. I've painted the top half of the walls using as always crown/dulux, crown in this case, after a couple of weeks on one wall under a slopeing ceiling the paint started to darken in runs from the top about 5ins down in what looked like runs, so I thought, plasterers PVA, from the ceiling skim, so i rubbed it down with an orbital sander sprayed "stainblock" on left it for 24 hrs then repainted with white,2 weeks on all seemed Ok so I applied the blue, within 1 week the runs had come back, not as bad but bad enough. After 30yrs of extensive DIY I've never had this before!!
Could I have some advice even if it's "you have done it wrong "

 

[JohnD]

bathroom

could it be condensation?

how long do you run the extractor fan after every bath, shower and emulsioning?

 

[saab9002]

I had thought about condensation, but rightly or wrongly discounted it because it was only on one wall and doesn't seem to feel wet, I've just repainted the offending wall and the door and windows are open and will be for a few hours

 

[JohnD]

no extractor then?

sloping ceilings are often poorly insulated

 

[saab9002]

Sorry forgot to mention it, yes I've fitted an extractor and it stays on for about 5 mins after using the shower or bath, this never happened before I redecorated the bathroom and we didn't have an extractor then !
Chris