I would never try to paint a car body with the inside of the body not masked off. There are just too many nooks and crannys for a speck of dust to hide in. When you start spraying the car there is alot of compressed air flying around.....and any little speck of dust is always gonna come flying out to land in your fresh paint. It never fails.
If one were to be doing a car that needed to be acid-dipped......and the inside of the body was bare metal.......I would spray the inside of the body first to make it LOOK like it was oversprayed, and then mask it off and do the paint on the outside seperately.