Personally I don't like Hammerite. It's too brittle & chips all to easily.
I've started to use a
chassis paint from FROST. It seems to stand up to knocks & bumps far better than Hammerite.
You'll need to remove as much rust & loose paint as possible & give the surface a light sanding down to act as a key for the paint. Apply a coat of primer, then the top coat of chassis black.
I used this chassis black to paint my 8 spoke wheels & it's standing up really well.