NaOH ( sodium hydroxide) in water solution is the standard method for removing annodized coating. Heavy duty oven cleaner, Red Devil Lye ( drain cleaner)all contain it. It doesn't take a strong solution and it will continue to attack the bare aluminim after it removes the oxide, so wash it off very well as soon as the oxide coat is gone. I recommend rubber gloves and chemical goggles as it is very difficult to wash it out of your eyes.

OB