Honestly, 24 or 36 makes very little difference. The lowest grit I use with my Rotex is 50 and I find that more than sufficient. In my experience you do too much damage to the surface with lower grits, which you have to restore again by even more sanding. With the Rotex, I do the rough work with Saphir 50 and then clean up with Briliant 80 and 120.