that is refering to a certain gauge of sand paper. You will notice that all sandpaper is identified by its "grit". The lower the number, the more coarse it is. A 40 Grit sandpaper will tear skin off your arm. A 1000 grit sandpaper will feel fairly smooth. You need to start low and work your way up. Get some 120 grit and start with it, gradually working your way up to 220, 320, 400, etc. until you get to 500.
Here's a link to everything you will ever need to know about sandpaper.
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