My painter washed the garage by hand (20 years value of chalking came off, in a few locations down to the bare metal). He did not powerwash for the reason that he didn't want any h2o driven up at the rear of the siding. Then he painted the main coat http://best-exterior-house-paint03692.getblogs.net/18517905/indicators-on-how-to-prep-aluminum-siding-to-paint-you-should-know