I just noticed this old post and thought I'd try to answer some of the questions asked here .
I've been airbrushing for about 8 years now and I think that one of the problems that Patrick may be having is from using lacquer that is meant for either brushing or with regular spray gun use . If that's what you are using , it HAS to be strained first to get rid of the larger particles that WILL plug up the very small nozzles in an airbrush , especially a flat or semi-gloss . I would suggest buying a lacquer that is specifically made for airbrushing , it is prestrained for you and comes in either water based or lacquer based . Beleive it or not the best gloss paint that I've ever used is actually a product designed for taxidermy use and is called "Competition Gloss" and is available from "WASCO" taxidermy supplies . Their link is below .
http://www.taxidermy.com/
As for consistency , it should be a little thicker than plain water .