I have a couple of levelling beams. One is long and does the initial levelling with 120 grit. I use belts for a belt sander- very consistent and tough. I use that to get the frets close. I then switch to a smaller beam with 320 on it to finish things off. I then crown with a diamond 300 grit file. Once left with that thin line on top of the fret, I go side to side with 320 grit or sometimes 180 to start. Then 400, 600, 0000 wool, buffer. That's it. The key here is your own strength to make sure each grit does its job fully. It's easy to move on before each grit has done its job.