Heard Hammerite is the best for painting the callipers, that true? What about painting alloys?

Hammerite smooth is best for painting calipers but not for alloys imo. Best getting a professional outfit to do it for you. The results will speak for themselves. Hammerite is ok on steel wheels, like on my land rover defender wheels. Get a bodyshop number from your area and either get them powdercoated or sprayed. That way you have a lacquer which stops corrision from the elements.