im working on a bid for a commercial building and need help. a couple of the sides are stained pretty bad with red clay up to about two feet. i'v e always used Bob's gutter bomb straight with a brush on vinyl with good results. i did a test spot the other day on the block with no problems but the metal siding really didn't do much after a couple applications and rinsing. is there anything out there that i can spray, dwell and rinse. im going to try a couple rust removers, and i think i may have some f8 lying around. any of you guys had any luck?