Has anyone else experienced discoloration in light-colored woods (maple, yellow cedar, etc.) when sanding with ultra-fine sandpaper? I seem to continuously get grey or black discoloration when sanding with finer than 400 grit sandpaper. I often have to go back to 220 grit and re-sand to get rid of it. What causes it? I use water-based sanding sealer and wood pore filler. Anyone found a way to avoid this discoloration?