Was trying to skim the ceiling last night but it seems I've had the bag of plaster much longer than I thought I had because it was several months past it's use by date.
I made up one batch last night and by the time I'd cleaned the bucket out, it had already started going off and was completely un-usable. I haven't done any plastering for a while so thought that maybe I'd got the mix wrong. I tried again with a much thinner mix, but again it only lasted a few minutes before starting to go off. It was at this point that I noticed the use-by date on the bag.
I'm just wondering what it is in plaster that causes it to go off like that and if there's anything I could have done to keep it for longer. The bag had never been opened, it was just one of those things where I bought it then kept putting the job off while I did other things instead. Obviously I put it off for too long.