Yesterday I showed a friend with a very rusty Honda junk yard bike  this trick.  He then went and got some very fine steel wool and a bottle of Turtle Wax chrome polish.  His muffler was very rusty and the aluminum worked fine on the smaller spots of rust - but it did leave the rust spot a little "proud" of the surface and you could feel a bump of rust.  His steel wool worked just as fast as the aluminum - but it took off the bump.  He did comment the aluminum was a neat trick, and his method is probably more expensive as it requries steel wool and chrome polish.....but it did work better.