Use some of those sandpaper sponges that you can get at any hardware or walmart store. You can get different grits for different stages of cleaning. You will be amazed. They are flexable and easy to hold in your hand. And ALWAYS use a can wrap so you dont scratch the can!!
How much rust? Big rust & lumpy stuff can come off in acid. Just fill a little dish with 1/4 or so of oxalic (some would recommend citric) and soak only the rim. Then flip the can over and do the other side.
Of course if you're just talking tarnish, leave it there. It adds character!
