Yep I agree with Arwyn! Years ago I was a fan of hammerite when it was probably the best option available at that time but its always been problematic to apply and you really can't guarantee there is no porosity! It also doesn't wear well. There are really good enamel and acrylic options available now and yes an acid etch primer really will prep the surface too, If I had to use Hammerite it would only be the smoothrite aerosol as the brush option is also patchy when applied!

There is also a rattle can option of an epoxy paint that I have found excellent you just pull and twist the ring pull on the base and shake to fully mix then you have about 20 minutes to apply. only downside is you can't save any paint not used in the can but boy does it do a good job!