you can polish just about anything metal, but like 245stroker said, you'd want to clear coat or powder coat anything that's not aluminum to keep it from rusting.
pc is better because it's more durable which is important on a 4wheeler.
some metals are def easier to polish than others. for instance anything aluminum is relatively easy (still takes a LOT of sanding and prep though) compared to polishing, say, hardened steel. for instance i polished my rotor on my motorcycle and i sat there and sanded w/ an orbital sander for like an hour before i was ready to polish. and even after that hour, i should've gone a little longer to get her to a full chrome like shine.
i suppose you could use the mothers polish like blasterboy said, but i'd recommend using actual polishing compound. i always start w/ red then do white.
you should use the search function, there are lotsa threads that talk about polishing.