If it dulls over time, it is bare aluminum. Reason..aluminum is easily oxidized by oxygen, to form aluminum oxide, that is what is removed by cleaners to expose fresh aluminum metal.
Rub a little "simichrome" or other aluminum cleaner on a little spot on the wheel with a white cloth, if the cloth turns black, it's bare aluminum. Then try the Barkeepers Friend and see how much easier it works! If the cloth doesn't turn black, and I mean unmistakably black, the wheels are coated and the polish won't help. The oxidation is under the coating.