Originally Posted by austin1989us
And why not on the inside?
Putting paint on the inside is only part of the process in dealing with interior rust. You need to scrape, wire brush, use a rust treatment, then paint. Painting on the inside will do nothing to seal leaks and prevent moisture from getting in the walls. That's an exterior job.
Elastomeric paints do a good job in reflecting the sun's heat away, since they are a blinding white colour. Their thick, rubbery nature probably also helps in that regard (slight insulating benefit). In addition, the fact that it is a thick, rubbery paint means it contributes to sealing the roof. I wouldn't trust it 100% for that purpose, but I'm confident it helps. You'll want to remove loose caulking, clean the roof, and reapply caulking to seal the roof properly. After that, many coats of elastomeric paint should keep things in good order.
Windows are also a point of water entry. You'll want to replace or re-caulk them as well... unless, again, this bus is meant to be a throw-away, short-term solution.
Nat is right about reading other build threads. You'll see discussion and great pictures that will answer all of these questions.
EDIT: I know... I'm guilty of spoon-feeding here.. For some reason, I just can't help myself today