I am having a square tubing rack built for my toad. A full roof area job. When it is finished here's what I Plan to do:
Scuff it with 400-grit. Wipe it down with acetone. Apply two coats of RED Rust-O-Leum primer. After which two coats of Rust-O-Luem black bed liner. All is available at Wal-Mart.
I don't know why RED Rust-O-Leum primer sticks so well to bare or lightly rusted steel but it does. It makes a joke out of other metal primers (even Cold Galvanizing paint). The bed liner is maybe the toughest paint I have ever seen besides DuPont Imron. I like it because it dries hard. Not gooey like asphalt based undercoating.
I've had good luck using twisted-bristle wire brush attachments. Big ones for 1/2" drills. The straight bristle jobs seem to melt before my very eyes.
WEAR FULL EYE GOOGLES!
From painfull experience I can tell you for some evil reason rust flakes will embed in the eye seeming easier than anything else. Requires a expensive trip to the opthmoligist. Regular safety glasses don't cut-it here. Use fully enclosed googles and keep them on afterwards while you wash your face and hair. It's easier to wear a cap. I cannot emphasize the eye protection enough. I was ignorant the 1st time, learned slow. It took until the 3rd agonizing doctor visit to pound this into my head (that glasses didn't work and debris in the hair made wearing googles alone insufficient protection.