Highly polished parts are primarily made of aluminum, e.g. Gearshifts, engine housings, silencers, water-cooled cylinders, intake manifolds and brake drums, etc.
Likewise, all stainless steel components can be polished to a high gloss. Stainless steel exhaust systems are somewhat dull in operation. Even after a repair of a stainless steel exhaust system by welding, a good re-polishing is necessary.
But certainly it is also possible to polish a crankshaft after machining.