Like mentioned, prep is the most important.

Also, there is a proper technique for spray painting. If you just shake and spray, like most people do, it will be an absolute mess.

I have seen a paint called POR-15 that is a really hard industrial coating. It comes out amazingly smooth when painted on with a foam brush. It is hard to work with and if you get some on you, it will be there for a couple of weeks. I have seen it used to paint suspension and undercarrage parts on several kit cars. I thought the parts were powder coated when I saw them. I don't think that there are many colors.
http://www.por15.com