Final Sanding

In an ideal case, a car can get body worked, primed with polyester primer, and then blocked out with 100, 220 & 400 grits. 
400 grit dry sandpaper, even on foam block, is borderline too aggressive for sealer to be on top of.  
The cloth sand ‘paper’ pictured is a perfect final step before sealer. It is approximately 400-600 grit, and leaves a smoother finish than 400 grit sandpaper.  Use it dry, wrapped around a soft foam block. 
This cloth paper is from Finishmaster, and 3M, Norton, and know sure many other brands have their own version.