There are actually lots of different types of sandpaper that range from coarse to extra fine grit. If you touch a piece of sandpaper and feel that rough, bumpy surface, you are feeling the grit. \
The grit of the paper can be very fine, which is 440 grit, or quite coarse at 40 grit. In between these two grits, you also have 220, 120, 100, and 80. \
Grit rated at 220 and 120, along with 440, are all fine grit, while 100 is medium grit. Coarse grit sandpaper is 40 and 80. \
Start with the finest grit of sandpaper, 440, and go up from there if the scratches won't buff out and the plastic won't polish up nicely. You probably won't make it up to 40-grit paper before the scratches have been effectively buffed out. \