I have cleaned a deck and there remains what looks to be like potting soil and charcoal black stains on the deck .Might be black mold still? The rest of the deck looks fine but the black stains are not removing. My mix was 3-2% bleach with 32 oz of F-18 in 8gals mixed then applied straight . Any suggestions? Also.I did do a test spot of about 5% Bleach to see if that would help but didn't.