Sometimes you can't help riding in the rain, etc. In my experience once the bare aluminum gets dirty it stays that way.
My cylinder head was stained so I used the spray can into a clean towel and foam Q-tip technique to rub on a very thin layer of aluminum paint. Big improvement and you can't really tell it has been painted. It has looked fine for 2 years.
I first asked my local automotive machine shop guy about putting the head in their solvent dunk tank but he said it wouldn't clean it well enough and he was the one that suggested the towel method. That's what they do for their show car engines.
I used some hi heat aluminum exhaust header paint and it does smell for the first couple weeks though.