Pure aluminum is very soft and will form it's own protective and sealed oxidation layer upon exposure to oxygen.
Since pure aluminum is so soft, most, if not all, of the aluminum found on our bike is an alloy with other metals blended into it. Such alloys do NOT form a sealed aluminum oxide layer on exposure to oxygen. Further, our atmosphere is NOT comprised of pure oxygen. There's also nitrogen, other gasses, as well as soot, pollen, and dust particles of just about everything imaginable, and some you can't. Many of the impurities in the atmophere will cause not only aluminum oxide but the beginnings of other corrosion, many of which do not seal the surface from further attacks.
Most paints intended for use on aluminum have an etchant chemical mixed in with it to remove, dissolve, convert, or otherwise neutralize small amounts of surface corrosion/oxidation on aluminum based metals. If your paint does not have the etchants in it, then it is recommended to first treat the surface with an etchant (look for Alumiprep).
Much of the success with paint is directly attributed to the preparation of the surface to which it is applied. There are certainly some random conditions where paint can properly adhere and seal properly. They key to predicted success is the control the surface prep for the paint that is to be applied.
Cheers,
