I wash about 8 aluminum sided houses a year. I don't do too many due to most around here have vinyl siding and if they dont, ive noticed several ,while going down the road ,having the aluminum siding removed and replaced with vinyl. It's been my observation around here that about 30 -40% of aluminum houses need to be repainted so make sure you always discuss this with your customers when giving them the estimate and offer to do a test area where the sun hits the house the most. That is gonna be your problem area. But I always make them aware it may have to be painted depending on the level of oxidization.