Aluminum does not rust, but it will discolor and develop a pitted or rough surface. Aluminum is affected by air pollutants. The surface will appear to have fine grit stuck to the surface, but it will not wipe away easily. If you observe aluminum storm windows and screens which have been exposed to weather for a couple years you will see what happens to aluminum outdoor furniture; the surface has oxidized.
Care will depend on the condition of the aluminum:
Regular Cleaning If surface is only mildly discolored, wash with soap and water, to which some mild household acid has been added; lemon juice, vinegar, or cream of tartar. Most water is slightly alkaline, so do not add ammonia, TSP or soda. Wash as needed during the season; wash at end of season. (Remember the darkening of aluminum pans when low acid or alkali foods are cooked..it blackens. When acid foods such as tomatoes are cooked in aluminum, it will become shiny.)
Special Cleaning If surface is pitted, polish with a soap-filled steel wool pot cleaner, rinse and dry. All steel wool must be removed or it will rust and stain the aluminum. Wash and dry.
There are also a variety of commercial cleaners used for aluminum surfaces on boats and automobiles which may help clean extremely dirty aluminum.
Do not use commercial cleaners on anodized aluminum.
A coat of auto wax, or a silicon spray will protect aluminum from corrosion and pitting.
Colored Anodized Aluminum: Use only mild detergent and water to wash anodized aluminum. Rinse well. The anodized finish resists corrosion.