I'm not an expert on paint finishes (I develope paint strippers!) but in a previous employ of supplying metal cleaners and pretreatment prior to paints, I've seen a wide range of paint performances from supposedly similar paints. As with a lot of things, preparation is the key, and paint (whether it be powder or wet, brush or spray) will struggle to adhere to improperly prepared metalwork. A wipe down with a solvented rag doesnt really compare with the multi stage cleaner - phosphate treatments used by the car manufacturers, but very few kit car chasses will be prepared this way. The paint on my Litton chassis fell of as soon as I looked at it, a waste of a few hundred quid. It stuck in places, but was still too thin. Corrosive elements will penetrate even the best, unblemished surfaces.
How come car manufacturers can offer such long warranties on bodywork, whereas 10-20 years ago Rovers and Fords would just crumble. Much of this is down to the use of coated steels. The closest we could get to that is by galvanising the chassis, although making it heavier and costlier, if done correctly it will far outlast a painted steel chassis. (It should also be painted or else white corrosion would take hold.)
But if steel it must be, then ensure that it is completely oil- and corrosion-free before painting. Some sort of etch primer would be applied next so that there is a chemical bond between metal and paint, followed by the topcoat. Powdercoats tend to be more durable, but if damaged may not hinder corrosion as well.
I could rant on, but that's the basics, in laymans terms. Any use?