Will this clean ext bare wood for painting and how long wait to paint?

Yes, Wet & Forget will clean green and black stains off of exterior wood surfaces to prep for painting. After applying, allow the wind and the rain to gently rinse and scrub the algae and mold stains away. You can speed up the process by power washing instead of waiting on the rain. Once all the algae and mold are off the surface, you're fine to paint.