Well, I can tell you it does make a mess so cover the floor! Coverage inside a boxed area is a guessing game. Not so sure it stops anything that's already started though.
I've had several old cars come into our shop that have been painted underneath and look great, but nothing done to the inside portion of the rails. I've gone to using an undercoating gun with a long wand and oil spraying the inside of the frame rails. Oil penetrates better than anything else and will slow the rust to a crawl plus the undercoat gun applies it very liberally........yes it will make a mess and drip for awhile, but paint will only lift and flake off after a short time. Like Daves69 referred to, paint cannot stop what's already started inside the frame and can hardly be seen. I hope this helps.