The back of our home is exposed to some hot sun in the summertime here in WI. I am starting to see more and more white spots (nickel size) show up that are in front of the nailheads beneath the siding. Is there any way to wash the siding or does this have to be repainted? The spots run straight down from top to bottom and they are spaced out between 1 feet of each other. These spots are noticeable on the entire back portion of our home.