I painted my siding about 20 years ago. I washed the house thourghly with a hose and a window brush on a pole - that was the hard job, I then sprayed it with a hand held Wagner sprayer (They had just come onto the market). I used Benj. Moore latex paint. A year or so later, I met a rep from BM and he said that they were developing a paint just for aluminum siding.
The siding needs to be painted again now, but I'm thinking of reshingling with white ceders.
I think that the whole thing is the prep - you can't knock 20 years.