It sounds as if you have some degree of experience but I can't get a mental picture of what you mean by "the paint is fine, it's the poor prep that shows through". This could mean a number of things from oxidation in the metal to sanding scratch swelling. Dupont has a good website with a LOT of info on what prep materials to use with what paint. But like everything else, if you're not trained in a specialty then you're doomed to trial and error. And body refinishing (like electronic engine controls repair) can make for some very expensive lessons.
Any photos of what you have?