If you can't stretch to a compressor etc get an electric spray gun, around £60, for a half-decent finish.
If Hammerite is rezcting with the etch-primer you need a blocking primer in between, thing is some are a bit nasty to spray at home. Why use Hamerite anyway, it's poo.
I'd go for a decent water-based acrylic, it will seal the surface just as well as Hamerite and shine a bit more.