I used Rustoleum black on my frame when I repainted it. All I did for prep work was put a wire wheel on the grinder, cleaned up the frame and painted right on te bare metal. The adhesion and coverage was great. I put the first coat on in a flat black, then used the gloss black for the finish. It turned out great!!! I also used Rustoleum gloss black on my dash pieces suck as the glovebox door and ashtray. It's a great cost saver and turns out a nice finished product in the end.
I have never used Hammerite so I can't comment on it. But I would definately recommend the rustoleum to anyone.