Further to the discussion on paints etc, today I finished rebuilding and restoring an old Crosman 600. Instead of using Powdercoat, this time I ordered a can of Eastwood's 2K ceramic paint in aerosol, their rally wheel 2k paint. I gotta tell you, this paint is da bomb! While was at it, and because I only used a part of the can and didn't feel like wasting the rest of the can, I painted a Sig P226 X5 too. Both guns turned out beautifully. I reassembled the 600, and there was absolutely no issue with build up. I had plugged openings with some putty, so that helped. Anyway, that's another option for you, and way cheaper than having someone else do it. The Eastwood 2K ceramic coating seems to be as tough as Powdercoat, and doesn't scratch.
Here are two photos of the refinished 600. The coating seems really hard, much harder than regular paint. Its only been a couple of days, and I think time may make it even harder, although I don't know if that is true with 2K epoxy type paints.