shaggs

G'day John
I just use quick dry enamel. Buy it in tins stright from supercheap auto. For stuff that gets resprayed every few years and banged to buggery you cant go past it. There are a few colour to choose or just tint yourself.
My truck and ute backs, chipper etc are done in the stuff. (Black underbody (cheaper and thinner than black paint)for "under the body")
Like anything, surface preparation is the key. Rust inhibitor etc.
I use HVLP (High volume low pressure) spray gun, gravity fed is the best. Dont spray in windy day 'cause as the name implies, quick drying.
About 3 good coats, wet on wet gets you a ripper gloss and very hard wearing. Some cans say you can add hardener for glossier and faster drying. I have used this once but the paint became too brittle and crazied. Use good thinners for paint, and crap thinners for gun wash.
Good Luck