Use 60- or 80-grit for aggressive, fast wood removal. Use 100-grit for all-purpose sanding and 120 or 180 for the finest finish, but follow power-tool sanding with hand sanding. Power tools leave hidden scratches that show up later. And always sand parallel to the grain.
Moreover, what grit of sandpaper do I use?
Coarse grits are typically in the 40- to 50-grit range. Medium sandpaper, ranging from 60- to 100-grit, accommodates some final shaping. Primary sanding of rough wood and the removal of planning marks on wood is often best done with medium-grit sandpaper. Fine sandpapers range from 120- to 220-grit.