we live in a house that looks like a log home but its not it siding.we have lived here 6 years and not put anything on the walls.i noticed about 1 or 2 months ago we are getting some dark grey black colored spots going the along some of the siding.(going not on down all the siding but along the siding)now the question is…is that mold?we did sand a small spot and it was white under.can i sand only the spot that have the dark spot and re stain all the wood?or do i have to sand all the walls?