There's a lot of threads on polishing here, various thoughts. I've always been told you should only use the green for stainless steel, so I've always stopped at white. There is one product a lot of people use for adding that final shine, it comes in a tube, but for the life of me can't remember it right now. You could also use something like Mother's aluminum polish.
I don't know about powdercoating, so can't speak to that. I have heard of people doing a clear coat over it, but eventually that clear coat will get foggy/off color. Plus you can never go back and touch it up, so I personally prefer to touch up the polishing from time to time. Personal preference I guess.