If you want to see air brushing that will knock your socks off, take a look at Raphael Schnepf’s work – his portraits on glass are absolutely gorgeous. His process is very unique and gives unbelievable results. His own site doesn’t really show the portraits well, but you can find some of them here: http://www.uroboros.com/artist_detail.php?id=42
and if you google Images For Raphael Schnepf, you will also see some of his portaits on glass work.
I am still amazed at the detail and the beautiful edges he gets using multiple layers of air-brushed stainer paints from Reusche’ and many firings.