If left outside unprotected Aluminum will oxidize. That is why a lot of aluminum parts are either powder coated or anodized. There is no easy way to clean that stuff of because it has attacked and changed the surface of the metal. The best thing to do is to have the polished parts repolished and the rough parts either sand or bead blasted. If you sand blast your intake make sure you clean it real well and get all the sand out before you install it!