Cleaning aluminum siding can be done with trisodium phosphate, or T.S.P. Several applications, in succession, of a mild solution of T.S.P. and warm water should get the job done. Mix approximately three tablespoons T.S.P. to a gallon of water. Stains and extra dirty areas may call for a stronger mixture, but do not go there until a third application of cleaner fails to produce results.

Use a soft brush to apply the mixture. My first choice is a truck or window-washing brush because they hold more solution than other brushes.
 
Apply the mixture to a small enough area so you can recoat it within ten minutes. Do not scrub while applying the first coat of T.S.P. Just get the solution on the wall so it can loosen up the dirt. Then go back and reapply a second coat of cleaner. The dirt should be running for mercy. If it is not, then apply a third coat and let it hang for ten more minutes before scrubbing.
 
Dirt that will not come off after all that is probably a stain that has worked its way into the finish for good. Now it is time for a choice. Do you double the mixture of T.S.P. to a gallon of water and possibly fade the paint on the siding, or live with the results you have attained so far? There are many quality variations of aluminum siding so you should try a small area and test the results for yourself.

Use a paint extension handle to reach high areas. They are available in many varieties from brand x to the very expensive. My advice here is to get a good handle; a cheap choice may be too flimsy to get any work done. In addition, the more expensive handles will be Fiberglas or some other non-metallic material that will prolong your life if you contact power lines.

Wear protective gloves, Bluettes or the equivalent, and some goggles. Do not get any T.S.P. in your eyes. If you get any on you, flush it off with plenty of water.

Water any vegetation around the house before using the T.S.P. mixture. Watered plants will not suck up much of the cleaner if they are watered first.