You can buy a product from any welding supplies trade shop or Supershit Auto sells it, it is called Aluminum Cleaner or Brightener, it is water soluble and brings mill or cast finish aluminum up like brand new, it is quite strong so be mindful of eye protection and gloves. It can react with some cheap Aluminum so try it on a inconspicuous spot first and you only need to leave it on for no more than one minute and hose it off with water. I have been using this stuff for 15 years as a metal fabricator and on all my bikes and its good stuff