I know the chrome part on the outside is just plastic. But what is the metal underneath? If it is aluminum, why can't I polish it with Mother's? It doesn't turn black at all. What can I do to clean the inner part and make it shiny?

Related to this, I have a CGS air intake and want to polish the top of my engine, but I have the same results. The metal polish does not turn black, like it should.

 

Steel rims under the plastic chrome-clad..

The CGS is clear coated, so it won't turn a rag black polishing like raw allow..