when i used to build model cars i would prime them
then mist them with water, and sprinkle them with salt
paint it, and rub off the salt
the top coat will come off with the salt, exposing the primer
perhaps you can try something similar?
or grease
ive done this before when rebuilding engines
put grease on the sections you dont want paint
once the paint dries, you can pull off the grease and then you just need to burnish the edges