Well it depends on how bad the problem is and what color your are sanding. Mine was white. The white gelcoat is really hard stuff. Good news is that it is pretty thick too. See my Shamu thread. I had to go down to 220 grit to get out the worst yellowing ghosts around where the old stickers were. I used a small sanding block where I could and sometimes just my fingers. Then up to 320 then 400 then 600 , then 1000 then heavy cut compound then fine cut. If it is not too bad, 320 may be fine to start with. It all depends how much you want it to glow. Once you get through the discoloration, the other grits are very quick, just to remove the sand marks of the previous size. JSG has a good link somewhere on the buffing side, but I figure you already know that.
Good luck!