Air brushes don't put out enough paint to do a good job on a bicycle frame. The best tool for the job is a detail gun. I use a Sharpe MGFHVLP. I think they retail for around $250. There are much less expensive guns on the market.
I just found one. I didn't pay that much. I got it used on eBay for $40 and bought a rebuild kit.
http://www.tcpglobal.com/spraygundep...value=SHA+7040