The grit is Aluminum Oxide. It is a synthetic red/brown-colored mineral that is tough and durable. It is great for both woodworking and metalworking. The most widely used grain.
General Guide:
60 – 80 Course (Fast stock removal)
100 – 120 Fine (Intermediate wood sanding or removing surface rust)
150 – 220 Very Fine (Final sanding of bare wood)
Micro Grits
220 Very Fine (Sanding finishes between coats)
320 Extra Fine
400 Super Fine (Final sanding of finishes or cleaning up machine parts)