Another vote for Rustoleum here . I've had some under my van for a few years, and no sign of corrosion yet. Thin it with white spirit and even brush painting (with a decent brush) looks good. Only prob is that it can take an eternity to harden properly...like weeks. I've never had good experience with Hammerite of any description, esp not Smooth. Rust always comes through.
Paradox, where did you source that tractor enamel you speak off ? it looks well good and i"d like to have some , ta 4 advice
Interesting comments about hammerite. I used smoothrite to paint the fuel tank on my 38 Citroen Traction; when I sold it 18 years later it was still like new. My only reservation about it was it becomes very hard and hence chips rather too easily.
I've used it on the tinware and other engine bay bits and I'm pretty happy with it. I've brush painted it on though so likely to be quite a bit thicker than using the spray cans.
Rustoleum Every Time ! sticks real well and comes up great either brushed , rolled or sprayed . Thinned properly with white spirit its a real winner . 8)