The challenge in preparing aluminum for powder is the oxide layer that is normally present on the surface. You did not say it, but most spray washers that are set up for multimetals use a fluoride additive in the phosphate solution to etch the aluminum surface and remove the oxide layer. This can be effective for products that are used indoors, but it is usually not sufficient for products that will be subjected to an outdoor environment.
Since there is no conversion coating applied, the surface can begin to oxidize very quickly after it is cleaned and dried, and the powder will not adhere if the part is exposed to moisture and sunlight. Aluminum that is treated in its own process without concern for running steel can be effectively prepared using a chrome seal rinse, a chromate or a dry-in-place nonchrome sealer designed for aluminum. Any of these processes will prevent the reformation of oxide on the aluminum and provide long-term adhesion.