I've got to agree with kik36...do the inside first. If you coat it with a little matte finish, and use light strength tape, it'll work fine...and man oh man I would be peeved if any of the interior paint got on the body....
I've used brushes too, because in reality, it is quite hard to notice the quality of a headliner in a model car...given the "not quite real" window glass.
anyways...just one of a ton of ways
-jonathan