Spray the entire door with window cleaner. Wipe down the door and brass to ensure that they are completely clean. Pay special attention to removing hard water stains on the brass.
Gently sand the brass with fine-grit sandpaper.
Tape around the finishing with painter's tape to ensure that you are not painting your shower door.
Paint the brass with an epoxy metal primer and let it dry completely.
Paint the brass with a very thin coat of metal paint. Let the paint dry completely. Add an additional coat if necessary. Let it dry completely.
Paint over the painted brass with a waterproof topcoat. Let it dry completely before pulling the tape off.