My painter washed the garage by hand (20 years worth of chalking arrived off, in certain destinations down to the bare metallic). He did not powerwash due to the fact he did not want any water pushed up behind the siding. Then he painted the 1st coat with the additive http://claytonturoj.aioblogs.com/18707265/the-5-second-trick-for-how-to-prep-aluminum-siding-to-paint