I stored some pine boards in my basement over the summer, and unfortunately the humidity was too high.  I noticed at the end of the summer that there is a dusty mildew on the surface of many of the boards that I’d cut and shaped for a garden furniture project.  It brushes off in part, but I can see that it has penetrated the surface of the board in places creating blackish discoloration.  Is there some kind of cleaner that I can use to kill that mildew and make the boards still finishable with an oil stain?  Thanks for any tips!