Aluminum develops a fine oxide layer when exposed to air with greater than 70% humidity and minuscule amounts of airborne chlorides - in other words, what naturally occurs in our lower mainland environment. This oxide layer is not uniform in most cases, over the entire surface of the part.
Some people use 600 grit sandpaper to make the surface uniform, then use a zinc chromate primer coat. If the part to be coated requires an automotive type finish, then you would use a polyurethane paint in order to seal properly.
Personally, I would use a two part process, such as Alodine, prior to coating. This involves an acid etch preparation, followed by the second part which passivates the surface, causing a thin inorganic layer, which can be coated with conventional primers and automotive coatings.
You can get zinc chromate in a rattle can spray bomb almost everywhere...even crappy tire.
For the latter, Dupont has a 2 part process called 225S cleaner and 226S conversion coating.
Have fun!
Csaba
