In the course of tarting up various bedraggled bits I tried both these aerosols in gloss black, on surfaces with most of the rust rotary brushed off, and treated with Jenolite, but otherwise unprimed. You'd think the Hammerite would be the best but it seemed a bit thin. Even after four coats you could still see the underlying metal on the edges of the shock mounts, for example. The cheaper Plastikote (c.£6 vs c.£9 for Hammerite) has a lot more covering efficiency.
According to the labels both paints contain enough VOCs to fry the eyes of every baby seal within a 50 mile radius, but Hammerite is far less obnoxious to use. The chemicals coming off the Plastikote make your head swim in seconds. Hammerite also has more of an enamel look to the finish. Plastikote is super shiny – something you may feel is out of place on an old Brit shonker.
The pics show the swing arm affected by Fee's original Cornish maritime rust, and the same item after a bout of rotary wire brushing and two coasts of plastikote. After a month it's dried rock hard.