I am polishing a marble shelf with pads impregnated with a resin containing diamond powder (it has some advantages over sandpaper, but otherwise works in a similar way). I used grit 50 to remove scratches and stopped when I couldn't see any more scratches.
Then I use grit 100, 200, 400, 800 (maybe more) to get a nice polish. But it's hard to tell the difference before and after grit 400 (for instance) by touching or looking. How do I tell I've spent enough time with this grit and I should switch to the next, finer one (in this example, grit 800)?
The final step will be a sealer.