I mixed some finishing plaster to skim the bottom of the wall where I had a skirting board before. I mixed the plaster (british gypsum thistle finishing plaster) at dropping consistency so it was fairly loose. Initially it was really easy to apply but 15 mins into skimming (or maybe quicker, it's hard to say) the plaster started curing in the bucket. I've skimmed walls before and I don't remember it going hard as quickly as this before. How long should it take before it starts curing? And what am I doing wrong?