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Just get a chrome alternator cover for it. While it "is" possible to polish the alternator, seems it would be more trouble than it would be worth to do it.
You would have to take it off, dis-mantle it, polish that pot-aluminum metal(which you have no idea how hard that is to do. ask me how I know, ugh), put it back together, put it back on, and then try to keep it polished and looking good.
Man, I just made myself tired thinking about that job!
I was going to have mine polished. The polishing droid told me not to; because it won't retain it's looks over time. He said to have it chromed; then all you do is wipe it off with a little Windex occasionally.