John Winters, the Shape of the Canoe, has a cogent discussion of surface friction, pgs 21-22.
The formula is K=14/v, K being grain size in mils, v being in ft per second. He states a grain size of 3 mills starts increasing friction at the 3mph pace.
Bringing the pace up nearer race speed, 10 fps suggests we need a cleaner surface than that, more like 1,5 mils to minimize skin friction. 400 grit paper is 23 microns=1 mil, 600 grit paper is 16 microns, hence .75 mill, so either will do fine in prepping a scratched hull.