In the final stages, the powder becomes < 20 micron and the mirror becomes sensitive to even the heat from your fingers. Dust that is invisible to the naked eye

In the final stages, the powder becomes < 20 micron and the mirror becomes sensitive to even the heat from your fingers. Dust that is invisible to the naked eye can leave deep scratches on the mirror. That video shows the kind of clean-room I meant. Gloves to reduce contact, container to avoid spills. I also used a cloth over my nose and mouth when dealing with the finer grits (in dry form). It need not be as clean as a fab, but it sure needs to be cleaner than a typical workshop or garage.