Design Nomenclature / Physical Model Making
Grit progression
Also called sanding schedule, grit sequence, stepping up the grits
Working through abrasives in sequence so each grade removes only the scratches the previous one left. Skipping a step leaves deep scratches hiding under a polished surface that reappear the instant finish goes on. Roughly halve the scratch depth per step; wet sanding at the fine end stops the paper loading.
In practice
"Take it to 400" means finish at 400 having passed through everything below it — not start there. The difference shows up under gloss, weeks later.