well Ed wasn't thinking of the inside. Mine did have a little rust on the inside I use stiff brush and diesel fuel. with the additives they put in diesel fuel now you will be amazed how good it cleans. If rust is really bad tho you may have to use some type of rust remover.
I would not paint the inside. It does need the rust removed. A product like evaporust, soak in cleaning vinegar, or electrolysis to name a few work to remove rust.