Wow... I thought that I was the only person to have that problem.
I tried buffing it, I had a company come out and apply hydrofluoric onto it, and never had any real great results.
All in all the buffing seemed to work the best, but in no way was it persect.
I know have a roller on all of our trailers, they apply a vinegar coat onto the aluminum first.
I mix the vinegar with a gelling compound, kinda goes on like the alcohol hand sanitizer, you know those little bottles with some gl type mix in it. It has a very limited life on the surface but long enough for us to apply sodium and potassium hydroxide, wash the Crete and rinse.
If you can get it off, keep me informed. I know that the 3M corp would love to develop a product of this type.