Our five-year screened porch has a beamed ceiling which we were told ny the carpenter to polyurethane once the porch was finished. He specifically told us not to put a water seal on it, just poly. It appears to have black dots that look like mold. Did we receive the wrong advice? Should we scrub the ceiling with bleach to remove the mold and then apply water seal and then polyurethane it again? Thanks for any advice!