Painting techniques - stright lines

for this, I direct you to our good friend Masking tape.  Put down a strip of masking tape (dont use regular tape, wil damage the paint) where you want the stripe.  It will give you a clean line without touching the paint udner it.  Search the forum for it, youll find a ton of stuff about masking tape, I couldnt live without it

2" blue 3M Painter's tape.  It'll hold a tight demarcation line without lifting anything underneath it.  Plus at 2" wide, you won't have to worry about overspray if you're getting a bit heavy handed with an airbrush...

Use colored decal paper...cut the stripes and place them on the model...why paint something so thin when you don't have to?