HOW TO PREPARE VINYL OR ALUMINUM SIDING
Wash the surface with a pressure washer or a cloth, and a solution of water and TSP. TSP is a degreaser used to wash surfaces before applying paint or stain.
If there is chalking (white powder) on the aluminum, wash it with an acid solution, such as pure vinegar, or a solution of 10% muriatic acid in water (always pour acid in water, and not the other way), or even a metal cleaner. Rinse well.