I'm not certain of the number - anything fine, maybe a light medium. Sand paper is cheap. I'd recommend some trial and error. When I did it I just had all kinds of various grits around.
Oh, another tip. Get a small funnel and coil your sandpaper in side it (you may need to trim it) and then you can stick the end of the funnel into a container and just swirl the stick around the inside of the funnel. You'll need to tap out the powder that builds up after a while, but this method is a lot less messy.