What's the difference between Rust-oleum and Rust-oleum Professional?
I dont know what the technical differences are but Ive used both for painting parts on my Jeep and have noticed that the Professional version gives a much smoother finish and self levels a lot better.  If you have the patience get the Professional version in the can and brush it on.  Looks a million times better than the spray can.

Prep work for both versions has been about the same, wire wheel, clean with mineral spirits, prime, sand, clean, final coats.