Aluminum, iron, steel, and other metals can be wrought (heated and hammered into shape), cast from molds (often in solid pieces), or formed into hollow tubing. Because most of these, except aluminum, are subject to rust, metal furniture is usually finished with layers of clear varnish, paint, or a durable powder coating.
Use a mild soap solution, and scrub, rinse, and dry as described. To remove rust stains or mold, sand the area lightly using fine-grit sandpaper. Wipe it clean, and apply touch-up paint if needed (available through the manufacturer) in several thin layers. Let dry between coats. Humidity may affect the paint, so it's best to work on a dry day.
After cleaning, apply a coat of quality liquid or paste auto wax with a lint-free cloth if recommended by the furniture's manufacturer. Use a silicone spray to lubricate swivels and glides on chairs, as well as ribs and poles on umbrellas. Inspect for rust or chips regularly, especially in hidden areas where the surface may be unfinished (even stainless steel can corrode in salty environments).