If the rust is under control to start with... or after repair a waxy rust preventer helps a lot to prevent rust in closed areas like inside door bottoms. Get it completely clean and dry then apply the waxy oil like the Eastwood stuff until the bottom part of the door inside is well covered and it runs out the water drains. It sets up overnight. It will also smell for a few days but does go away.

http://www.eastwood.com/eastwood-heavy-duty-anti-rust-in-amber.html