Why can't I sand a pencil mark off? I'm working with hard Maple, Walnut, and Padauk. I put a pencil mark over the wood as reference for my sanding stages. I sanded the pencil mark off of the Walnut & Padauk, but it is not coming off of the Maple, even with 100 grit paper on a belt sander. Does anyone have suggestions, to get it off?