It ok but if you want a professional finish they have to be sprayed, I hammerited plenty when I was young and they looked ok but I'd never think of it now, if your going to go to the hassle of painting and masking them up just get some high temp coloured spray at least, it will give you a better finish than hammerite at the end of the day
Best advice I can give is, you your just going to wire brush the rough off and paint, then Hammerite is ok, any more prep and I'd spray them