Well, keep in mind that full scale camo was done free hand most of the time anyway, so masking often makes the camo look too "sharp", a fuzzy edge is a good thing here.

Is your airbrush single or duel action? Mine is duel action, allowing me to control the size of the line drawn easily.

When I spray, I find I get spattering if I use too much air pressure or not enough air pressure, or paint that is too thick. I also get it if some paint has partly dried inside the gun, and clogged things up. That's actually my #1 problem, keeping the thing clean and working with out any obstruction inside.

Oh, and if I screw up and go outside where I planned to, I just adjust the plan a little. Unless you're doing a competition quality scale finish and trying to match every curve and wiggle of a particular full scale plane, you can get away with a lot of "faking it" and still get a plane that 99% of the guys out there can't tell from exact scale anyway.