Ive used hammerite on many domestic external applications (patio heaters,garden furniture) - and on bikes for swing arms, centre stands etc., and always found it very durable, lasting many years without degrading: but Ive never used it on engine casing or blocks before.
But, surprised you've had issues with Hammerite: I know others have used it on engines with good results.
On engines I've previously used Simoniz Silver & Black Engine Enamel (good upto 800C) - and found the end result very good, so I generally stick with that.
Cheers Tim