I used hammerite with their primer as well, on my wishbones. A few coats of each were sprayed on. I dropped one of the wishbones about 2ft and a huge lump of hammerite flaked off
I am now using a paint called QD90. I explained to a local trade paint supplier what I wanted it for and he recommended it. White spirit is the solvent, not the silly expensive hammerite one.
It is a much more flexible paint - I beat a test piece up with a hammer and was quite impressed
Sean