Do you sometimes get that dark spot right in the center when you're airbrushing? That's called "spidering," and in his airbrushing class at the National Capital Cake Show in Annandale, VA March 25-26, Edward Frys offered some troubleshooting tips on how to get started airbrushing and improve current techniques.
There are three elements to airbrushing that you must pay attention to: pressure, distance and duration. In order to master all three in perfect combination, try practicing on paper first.
The spidering effect happens when you apply too much pressure to the airbrush. "The air stream is always constant," Frys says. "Pulling the trigger only does one thing—it introduces more color." So if you take care to go light on the trigger, you'll be less likely to see spidering.
Another tip Frys pointed out is that the same duration/pressure at a further distance will make the color lighter, the higher (or further away) you go, the lighter the color will be.
If you're covering a cake using airbrush, there are few things you can do to avoid dimples—the dots that can happen at the starting and stopping points. Dimples occur when you heavy-handedly start and stop. If this is a habit you can't break, then make your starting and stoppign points outside of the cake. While this is a method that can work, it can also get a little messy—and sloppy. Ideally, the best thing to do to avoid dimples is to learn to go light on the trigger, and keep your hand moving in one fluid motion across the cake. Start moving the airbrush from outside the cake, then apply gentle pressure as you approach it. Then release as you reach the edge of the cake, and continue the wave of your hand.