If you have really rusty cover plates you can attempt to bring them back to life with a little WD-40.  Soak them up a little then give your plates a good buffing out with Semicrome Metal Polish.  You may end up with small pits but you'll have to look really close to see them. It is a very economical way of cleaning up your covers.  If you have any other suggestions please feel free to email me and I'll be happy to post your comments.