Looking great Dave 
Instead of cardboard stencils -I used Clear Contact on all curves and shapes,a lot thinner than cardboard and seal all edges as Tony mentioned. On all straight lines I marked with pencil and used Blue masking tape,sealing the edges the same. The pencil rubs off easily when dry with a pencil eraser.
What type of paint are you using,acrylic,enamel or water based enamel ?
I found using the Roller gave a great result - Cheap and no mess.
Kev