I guess that's the brush on type. I might buy a small tin and try it out on something else than the frame first. Supposedly the aerosol spray paint version is not as good, and the hammered effect not quite as deep. If using a sponge type brush it should be easy to get a good even finish.
This guy might have a tip or two about Hammerite
https://www.lfgss.com/comments/13537377/