My window trims are coated with exterior Polyurethane over bare wood. Since installing energy efficient pleated shades, each morning after sub-freezing overnight temps, there is heavy moisture on the windows at the edges where the glass meets the trim. A quick wipe with a dedicated towel cleares the moisture but over the last two years or so, mold has grown where the window meets the trim. This mold does not occur on windows when the pleated shades are kept in the "up" position. So, I am going to keep the shades in the "up" position during winter months. I want to eliminate the stain and apply 3-5 coats of new Polyurethane for continued protection. The question is this: How do I eliminate the fungus which seems to have grown under the polyurethane and stained the wood? Bleach? Scrapng with razor, etc. ?