Our house is about 80 years old with cedar shake shingles. They have been painted twice, the last time about 12 years ago. The paint is flaking off so we can see the gray. We would prefer not to paint the shingles again. Can we let the paint wear off, perhaps with the help of a palm sander, and then apply some sort of preservative? Or do we have to have it re-painted because it was once painted. We have had many recommendations including power-washing, painting, etc.