Greg, It really depends on the type of application you have to determine the best pretreatment. It needs to be clean or organics and inorganics, and relatively free of oxides. Also consider chromates being a heavy metal with disposal considerations weighing heavy. This will clean, etch and provide some deoxidation. There are various proprietary products available.
Iron phosphates with florides incorporated with provide a light aluminum phosphate coating.
Very rough surfaces still have to be cleaned. They also require a lot of paint to provide a nice smooth coating. This can become an expensive proposition. For ones and twos, grit blast it and immediately paint it with a primer for aluminum. Many paint stores sell one.