Depends on several factors. With a tripple 0 brush, and a steady hand, you can "draw" pencil thin lines. Depending on the size you require, you could use something we called "frisk film" which basically if a weakly sticky thin plastic film where you can cut out whatever figure you need. Stick that to the model and just spray paint over it with the air-brush. It will mask out your design. It's a kind of reversed decall. You stick the outlines on he model and paint the actual drawing "through" it.