Hammerite has two varieties the old type (like the cans on my workshop shelves) which smell of thinners/acetone and dries far too quickly and the latest incarnation which appears to be oil based and dries a little too slow for my liking.
The two types must not be mixed.
I have tried thinning the stuff using ordinary celulose thinners and Hammerites own thinners but neither seem to work very well OK for cleaning the brushes but not to thin to use.
If you wnat a hardwearing smoothish oil resistant hardwearing finish you could do worse than use Tractor paint “Tractol” or “Sparex” to name but two dries quickly enough not to run but slow enough not to drag or leave brushmarks.
regards
Paul