I have seen and heard of powder paint with rust issues. It's impossible to have a really sealed paint coat. So rust can and does creep under the paint. Without a primer/sealer base, powder will let this creep occur far more then classic wet paint (with primer/sealer). Cable stops, bottle bosses, cable guides are the big points of breaking the paint's coat (and then show up as rust spiders).

So for most powder coating that I've know about it's about the cost and not the rust prevention. Andy.