When you want to color /clear sand then polish in order to make your paint job flat and glass-like there are several things you need to keep in mind and this is one of those things.
You’ll usually want to initially cut the surface with a coarse grit that will remove the surface irregularities then go over it with a finer grit to remove the scratches left by the coarse grit. After sanding you’ll polish the surface to bring up the shine.
If you sand with a coarse grit you have scratches (like shown in figure 1) that will be difficult to remove by buffing so it’s usually best to sand with a finer grit before polishing. When you begin this second sanding process it will appear to be finished quickly but you will be taking off the tops of the sanding scratches as shown in figure 2. If you polish before you sand all the way to the valley floor (figure 3) you will see glossy paint with fine scratches in it.
The trick to a good sand and polish job is to sand the surface flat using coarse and fine sandpaper then polish as shown in figure 4.