To add to whats been said, you definitely dont wanna sand on any that have metallics, pearls, etc, and if you MUST, then you also need to lay another coat. If you use an activator with basecoat, in certain circumstances it can be a lifesaver in case of any accidental sand-thrus, as it may be able to keep you from having to redo an entire panel.
Russell, you are correct, sanding on a solid color with no metallic would be fine. However, Id still prefer BC/CC over SS. This is just my 1½ cents worth...