I've had good results with the Rustoleum Professional Enamel line....but it takes a lot of cure time before it becomes durable. I painted an off-road style bumper with it and got a few chips from handling in the first week or so after painting. After about two weeks I could hit it with a hammer and it wouldn't chip.
It was bare metal. I degreased it with some brake cleaner, sanded it a bit with a scotch-brite pad, then used POR15's cleaner/degreaser followed by POR15's Metal Prep. The Metal Prep etches the clean metal and leaves a zinc phospate coating to prevent future rust and improve adhesion.
Then I used some Rustoleum Professional enamel primer followed up with their Professional Enamel gloss black top coat. I painted it 12 years ago and it still looks perfect.