A great tool for grinding a seat is the paint scraper by 3M. The name is decieving. It's a round wheel about 3" in diameter and about an inch thick made of wire mesh that's impregnated with an abrasive. All home centers carry them. They're about $5. You use them in your drill. They're perfect for carving the rounded curves of a seat. For large flat slopes, I use the nose of my belt sander. Then you can finish it with some hand sanding.