In reply to the Brownell's garnet paste: Yes, the 5 micron would be around 4500 grit. When you look at 0.25 micron that is really about the equvalent of 100,000 grit and 1/2 about 50,000 to 60,000 grit.
Pertty small. Many bacteria are 2 microns in size. We polish filter wine down to about .50 micron just before bottling. A red blood cell is about 8 microns.