I used CLR to remove tree stains from my eaves before selling a home I owned. Since this can soften some paints, try a small area first before damaging something.
Trouble with aluminum is that it will oxidize quickly in normal atmosphere, so it won't really stay oxide free very long. But as far as removing the oxide there are several options:- fine sand-paper-...
Grab an empty spray bottle. put a bit of soap in, and abit of shampoo. fill it with cold water. spray the mixture onto the spray and rub gently with a cloth and the stain fades completely so there is...