jj -
The raised paper thing works well, but there's an easier way to do it. I keep alcohol pads on my work bench to wipe down parts or help remove acryl aint. When they are dry, they can be cut to shape and taped to the model. That have a soft edge that give a nice soft, and apparently scale correct line between A/C colors. Works great with an air brush, and should work with rattle cans. Just remember to spray perpendicular to the surface.
BTW - this was in the Readers Tips section on the last FSM. I guess I'm allowed to post it here, as I'm the author!!!
Archer out.