Polish

Step 4 uses a polish instead of a grit: most often aluminum oxide, sometimes cerium oxide or tin oxide. The particles are far too fine to remove scratches. They only bring up the shine on a surface that is already smooth.

Cleanliness matters more here than anywhere. One grain of silicon carbide carried into the polish scratches the whole batch, so scrub the stones, the barrel and the lid seal, or better, keep a barrel that is used only for polish. Run for 7 to 10 days.

If the stones come out dull, more polish is almost never the answer. The usual cause sits in an earlier step. Why tumbled rocks are not shiny walks through the causes in order.

Ready to move on when: Dry stones shine. If they shine only when wet, the surface was not ready for polish.