This thread on air brush painting (and a few other subjects) in the How To forum is one of the best I have seen. It is also in BNSF7173's personal blog. Many other excellent tutorials are in the blogs; we will eventually reference many of these TB blogs in the How To Forum to place several modeling ideas in one place. Additional search terms: respirator mask air brush airbrush paint acrylic testors custom
I can personally attest to Jim's method. I especially learned a bunch just from paint prep (thinning) and shooting (pressures, technique) the paint proper. The results had improved nearly overnight!