Cleaner removes what is on the metal. Polish removes what has happened to the metal. Using them in the wrong order is how people scratch aluminum and chrome.
Polish removes rust that sits on the surface and does nothing for rust that has eaten in. Here is the 30-second test that tells you which one you are dealing with.
Chrome plating is thin, so scratches cannot be polished out later. Here is how to prep, which products to avoid, and the technique that keeps bumpers swirl free.
Aluminum turns grey and chalky because of a layer of oxide, not damage. Here is how to remove it by hand, which surfaces give the most trouble, and how to stop it coming back.