Practice on paper with your airbrush, I would try reducing your paints and set your air pressure to 45-50 psi. Another thing is keep your airbrush clean even when your using it specially your needle, the tip of your needle will tend to load up and dry out. I am constantly wiping this off because if you don't it will unclog onto your art....not good.
Contact me anytime if you need more tips and thanks for your comments!