Originally Posted by Everett Abrams
The correct method taught in the industry is to wet the surface well from bottom up and then apply your cleaner from the bottom up. Allow for the proper dwell time and rinse from the top down. Chemicals can leave streaks on surfaces when not properly applied. When estimating a job you should perform a "finger test" and if there is oxidation you should determine whether the job can be done and then inform the customer either way. We also always ask how old the siding is so we have more information as well.
+1 it is widely accepted to soap/apply chemicals from bottom up and rinse top down. Helps alleviate issues.