The Roof Cleaning Institute Of America Training & Certification Forum
"Find A Certified Roof Cleaner Directory"

Not much you can do about oxidation. All you can do is let the customer know you will do your best but that it will not look new. For that you have to repaint.

I found that a strong house wash mix works pretty good and then apply a coat of plex master after the cleaning. You can get it at envirospec. Now this is for painted metal I have not cleaned a galvanized roof and I would not use SH on a galvenized roof. The contaminates are different so you have to clean it differently. Think of it like a large gutter.