A great resource for airbrush techniques that you can apply to miniature painting is plastic model sites and magazines. Check out Hyperscale.com and Fine Scale Modeler magazine. I do WW2 model airplanes and miniature painting and techniques from both schools can garner great results.
One thing I can say is I only use plastic model paints in my airbrush and I use Reaper / Citadel paints when brushing. That can be expensive though when starting from scratch with a new line of paints.
If you're getting a 'powdery' finish it sounds like you're either spraying too far away, using too much air pressure, or a bad thinning job. Acrylic paints are meant to be translucent so you will get best results with several light passes. Thin your paint way down to the consistency of milk, then slowly raise your air pressure from zero PSI until you get a consistent spray of paint. You're going to want to be painting from about 1-3 inches away from your piece. Practice, experiment, repeat. To get some really spectacular blending coats try using Tamiya acrylic paint thinned with their own or Mr. Color brand Lacquer thinner. You'll get some really nice, smooth, translucent blends perfect for the technique you're looking to do.
I haven't been able to get good results using Citadel or other miniature paints through my airbrush but I haven't really tried all that much. A drop of Liquitex flow-aid to an A/B mixing cup of acrylic paint may help things a bit. Always use your paint's recommended thinner.
I'm assuming you're painting vehicles of some sort? Try using Tamiya, Mr. Color (Gunze), or Testors Model Master / Poly Scale paints if so. Again, use the recommended thinner - people say that Tamiya thinner is just isopropyl alcohol but it's not; it has some special Tamiya fairy dust in it that works wonders.
Happy painting!
