Is it rust as in the copper color, or corrosion - a white and red color. Ranex needs a bit of rust to "react" and it sounds like it could do the job. With older transformers, I remove them and hit them with a wire brush - removes "everything". Since the plates are already black, I don't paint them. With newer transformers, I'm not so sure - but check out John Wart Jr's CFTBL restore - I'm sure it is covered.
http://aussiepinball.com/index.php?topic=3124.0
