I like to finish to 400 before blasting, this is partly because I have found that the finish prior to blasting can affect how fast it will rust, and partly because I like to use very fine media, essentially for the same reason.The media I use will not cover scratches much coarser than 220 or 320 grit.
Like Justin, I take the flats to 400 and will hit the bevels with a fine scotch brite belt. I use 80 grit glass beads and probably could get away with a 220 finish.
I haven't tried to bead blast CF. As it's much softer that Micarta or G10 my guess is you'd have to be careful not to take off too much material.