I read in the book "How to Restore your Chevrolet Pickup", that the best way to handle rust on the inner door surfaces is to use a rust neutralizing product. It says that products like Zintex from T.N. Cowan, converts rust to an inert coating that seals the metal against further rusting. The book goes on to describe that the metal must first be clean of grease and paint and if possible, loosen any rust scale with a wire brush. It says that the best way to use it on the inside of a door is to take the door off the truck and lay it down with the outside skin facing down. Next you poor the rust converter inside the door and then the pick the door up to spread it around by tilting it at different angles. Once you have done this the instuctions call for washing the metal and then sealing it with a coat of paint. The best way to prime the area is by using the same method used to apply the neutralizer. Then just let it dry and repeat with a finish coat. Of course you should read the directions with the product but I think I got everything in here that the book said. By the way this book is an excellent book with lots of little tricks like this. I really like it as I am just starting into my first project. The title again is "How to Restore Your Chevrolet Pickup", and it is by Tom Brownell. I hope this helps!