It really depends on the type of bird.... some birds love to chew on bars, some don't. Truth is we use Krylon spray paint every year to touch up our outdoor aviaries (they're black). After our house fire we had to go through what was left of the cages.... most were thrown away, but we had one big double cage that I sandblasted, re welded in a few places, and then we spray painted it. We only use it as a back up cage ( for birds that get dumped here). But it hasn't caused any trouble...
As far as powder coating and toxic materials go, I know of one cage distributor who gets his cages from asia.... and lotsof birds have been reported sick if kept in those cages. (If you are afraid your cage might be one of them, message me and I can send a photo of the latch (which is how the cage can be identified.) (I have one here that we keep outside and use for rehabbing raccoons.)