My home has aluminum siding that is 23 years old. Last summer I washed it for what looked like the first time in many years. Fortunatley, the paint did not come off so repainting was not necessary. At the recommendation of a couple of articles I found on the net regarding aluminum siding washing, I rented a high pressure sprayer and went to work. The result was that regardless of how careful you may be you will get water under the sideing, in to the soffit vents, behind the j channels at the windows and doors, etc. The most noticeable result of water behind the siding (ignoring even possible damage to the sheathing) is that what ever dirt has collected behind the siding will run out at the seams and weep holes to leave visible stains on what you just cleaned. It was simple impossible to wash the house this way.
The most effective cleaning method I found was a 16 foot extension pole with soft brush (the kind you would use on your car), a 5 gallon bucket of hot water with a scoop of laundry detergent and a cup of bleach, and your normal outdoor, low pressure hose. Do one side of the house at a time by first applying low pressure water to the siding (being careful to avoid soffit vents or spraying directly into seams, etc.). Then start at the bottom and clean up liberally applying the cleaning solution - do not let the solution dry. Rinse the house as you clean and one more time once the side is completed. Be careful to avoid getting this solution on plants near your home since it contains bleach. The result is that what was once very dingy, dirty, and sometimes moldy siding now looks like we either painted it or replaced it. I will tell you that this method will take much longer than the high pressure sprayer but the results are outstanding.