I just did the tow hooks and the trailer hitch on my XJ, with Hammerite. Used the new water based formula for the front and the old formula for the back. Been holding up well so far. The water based (new formula) is kind of a half flat or satin finish, seems to be pretty durable, I painted over my chromed tow hooks (after sanding) and the strap hasn´t rubbed the Hammerite off of there yet.
I did the whole frame on my 76 Dodge PU, with rusty metal primer and Rustolium gloss black, been on there for about 15 years, no problems yet. Though I did a serious job of prep, with a mild sandblasting and a rubdown with aceton.
I used rattle can Rustolium rusty metal primer, then brushed on a light coat of gloss black (a section at a time) then lightly sprayed over the brushed on gloss, with spray gloss, removed most all of the brush marks and left a medium thick, fairly smooth coat of paint on there. Took most of a hot summer to cure right, takes awhile to get really hard.