I only paint with spray cans, so if you're planning on an automotive quality paint job, you can stop reading now...
First, I've tried any number of brands of paint, and haven't found anything that's easier to use or outperforms Rustoleum.
After all the work on the frame is done, everything is ground down and you're ready to paint, soak a rag in mineral spirits and wipe the entire assembly down. Let it air dry.
Then, hit it with a good coat (or even two) of Rusty Metal Primer. Yes, even if it's not rusty. The RMP gives you a tough base coat with plenty of "tooth" for the color coat to get hold of. Let the primer dry to the touch. IIRC, the can says "Recoat in 10-15 minutes OR wait X hours" meaing recoat while it's still tacky, or wait until it's completely dry.
Then hit it with a couple of coats of whatever finish color you like.