Ok, I'm painting some wooden window frames and have justed started on the top coat. I'd undercoated using Dulux undercoat and then started on the top coat after letting the undercoat dry for ample time (actually a couple of weeks as I'd lost interest as you do

).
All was going well & I'd got to one particular section when after a few mins the paint started to separate from the undercoat & collect in "ripples". All the other areas were fine - but this was a flat/horizontal section - whereas the others were vertial with no probs.
The paint is Dulux satinwood quick dry. It was an unopened can & I suspect that its age may be an issue as I bought it 4 years ago and never used it. I stirred as per the instructions.
It eventually covered as it should, but as it is a quick drying paint the finish was just starting to go off when I'd got the coverage right.
This seems to be a water based paint - at least that's what it says to wash your brush in anyway.
I'll have another look tomorrow when it's properly dry & try a 2nd coat on the affected section.
I've always used Dulux over the last 30+ years and never had a problem before.
Any ideas?