I'm not a professional painter by any stretch of the imagination, but this is what has worked well for me.
1: Scotchbrite pad with mineral spirits to buff it up a bit and knock off any rust. (my parts have been new unpainted.)
2: Clean off any residue and grease with Simple Green and hot water. Wear latex or nitrile gloves to keep fingerprints and body oils off surface.
Make sure you have a place to hang everything before you start. A rockhard cage has lots of parts. I put a ton of yellow earplugs in the threaded holes.
3: Rustoleum clean metal primer.
4. Enamel paint. I use Ace Hardware brand because the can is a bit larger. I like their satin black finish although I used a hammerite finish for my cage.
I also removed and painted the main hoop to windshield side bars and with a lot of masking tape and painters plastic, the factory cage.
Good luck.