I bought some 6' railing segments from Lowe's last year. They have vertical metal poles secured by horizontal wood (probably pine) beams on the top and bottom. I stained the railings with a Cabot outdoor stain and sealed it with Cabot poly. After 2 years, black stuff - assuming mold - has developed on the bottom beam. I've tried bleach and it doesn't budge. I'm really upset because I did what I though was right, the railing looks great, but now I have this black stuff to deal with. Any helpful advice is welcome!