If you want somewhat soft edges of your demarcation lines just use your airbrush. If you want hard edges  "frisket" film is an excellent choice, fairly cheap too, that you can cut the pattern and stick to your surface. If you use a sticky paper or film just make sure you get all the edges down tight to prevent seepage underneath.

I always find an airbrush gives to much of a soft edge, unless it's a really big model. The plastic modellers do it this way, cut the shape required from thin card, then attach it in position with Blue Tack, so it stands about 1/8" proud of the surface, then airbrush the colour keeping the airbrush at 90 degrees to the work, on a low pressure! It results in a fine soft edge.

Cheers