A belt sander like that will take off way too much material in the blink of an eye. A lot of TDPRI'ers use the Ridgid ROSS from Home Depot with a spindle sander and belt sander that are stationery. One of the most useful tools in the shop. The belt sander on that is what you'd need if you want a proper one. Use a flat block of wood and sandpaper. You have it down to the line already. I've done the same thing with a spindle sander that you have and a few minutes of hand sanding will smooth it all out. Mark some pencil lines in your divots and sand until they disappear.