When painting the calipers don't underestimate the length of time it takes for each coat to dry. The Hammerite tin says about 2 hours per coat, but it takes longer if it's cold and wet.
I tried painting mine at the weekend, and didn't have axle stands so could only do one wheel at a time. It took about 4 hours to do just 2 coats on ONE caliper - and to be honest it could still do with a third coat to get rid of the last bits of metal that are still showing through!
So the end result of a Sunday afternoon's work? One red brake caliper.
Think I'm going to have a go at finishing them off next weekend, but need to start early, and hope it's a bit warmer!