The long sanding board is the key tool for a wave-free surface. Follow up the filing by sanding with 36- or 40-grit sheets in the long board. The coarse paper will cut and level, while a smoother paper will float, leaving dips and humps. Work the board at a slight diagonal and criss-cross the area by changing directions. When the filler is level and uniformly sanded, switch to 80-grit to smooth the heavy sanding marks.