Kinda like what oil or wood to use question. Those will work, I prefer to use cut off and 1/4" thick grinding disks made for concrete work. They cut well, last forever, and leave the surface rough. Perfect for the new resin to bite into. The sanding disks 40-60 are better suited for a quick rough up on newly set layers to knock off any chunkage and make better transitions for the next layer. Just my $0.02