Yesterday I painted a cornice with 2 coats of Johnstone's white matt emulsion. Today when I looked at it there were some brownish droplets on the cornice and the paint hadn't dried. This only occurred in a few places - the majority is fine. Any ideas what the problem is and how I solve it. I can wipe the gunge off but will it just reappear if I re-paint? The cornice is pretty old and I'm sure it would have been painted before. Admittedly I was rushing and it's possible the first coat I applied wasn't quite dry (I'd left it about 3 hours), but surely this wouldn't cause the brown stuff?