Over lap technique for airbrushing

Try using a paper mask (piece of paper cut to the mask shape you want) and stick it to the surface using balls or worms of either silly putty or blue tack (poster tack, that stuff that holds up a poster on a wall without putting holes in it, kind of blue or white puttyish stuff).  The farther away from the surface your paper is, the softer your line will be, the closer to your surface, the sharper your line will be, both offering a very slight amount of overspray.

With a regular brush, soft edges are very hard to generate.  Soft edge camouflage is one of the reasons many folks first buy an airbrush.  As the above post says, you can get a softer edge with a spray can and proper masking.  But to do it with a paint brush takes a real artist and lots of practice.