Getting paint to stick to nude aluminum, long term, is difficult. You first have to clean (neutralize) the aluminum, everyone has their own secret formula, and then prime with a good zinc chromate primer. The issue is, aluminum grows its own oxide layer to protect the surface, similar to what stainless does, however it's more aggressive.
Have you ever tried to polish aluminum and it seems like no matter how many times you polish the metal the cloth always turns black? That's the oxide layer that the metal continues to grow and it always will. Just the nature of the beast. So the best methodology is to neutralize the surface and then apply the primer.
The exception to this rule is if the material surface has already been chemically altered, typically through anodization. Then it's a quick scuff and squirt.
