Dull the aluminum with fine wet-or-dry 440 sandpaper (used 'wet'). When it's completely dull, wash it well with white vinegar (neutralizes the aluminum). Spray on several VERY LIGHT DUSTING COATS of Zinc Chromate primer and let dry for several days. Then coat with your final paint. (Do a test sample to see if this is compatible with the type of paint you are finishing with - you didn't say which type you are using.)