Originally Posted by Amesja

In my experience when you polish aluminum it doesn't stay shiny all by itself. Aluminum oxidizes fairly rapidly when exposed to air and turns a matte gray. The more it was polished the longer it takes. So when it does go flat it starts a bit blotchy at the areas that didn't get quite as much of a mirror finish. Shiny aluminum from a factory that has a flashy finish always has some sort of lacquer over it to seal it off from the atmosphere. If you polish it you might just clean up the lacquer clear coat or you might take it all off and expose bare aluminum under that. Mothers does an OK job of sealing it for a medium to short while but if someone washes it later with dish soap or other harsh detergent (even vinegar which strips wax right off) it'll be gone and then oxidation will set in.