I've noticed a lot of complaints about "spitting" airbruhes. Here are 2 rules to help ameliorate this pain:
1-Always start spaying away from your workpiece-a piece of scrap cardboard in between the model and airbrush works great. Eliminates those start-up spits.
2-Keep a soft sable brush handy to periodically wipe the tip as you work-paint usually builds up on anything but the perfect needle.
Remember keep it clean, clean, clean!