Start with 120 grit Drywall Sanding Screen to get flat, on the floor on a piece of marble or granite tile in a cookie sheet, then progress to 220 and 320 wet and dry, once flat. Use your body weight and lots of water.

The 220 and 320 will lap off any grit that come off the 120 screen.

Should only take a few minutes, you are just not using low enough grit, or as said a 320 ish diamond plate.

Chef’s Knives to Go, has a real nice 140 grit 8X3 plate for 30 bucks that will make quick work of lapping to flat.