Stock surface is 4000 + polish.
While sanding without a spinner can be tedious and irregular, it can be done.
I'd use a 2000 pad with lots of water to start with.
2000 is pretty dull when compared with 4000+polish, so it might not be a bad use point. You can always try 4000 if the 2000 is too early.

When sanding, I'd suggest having some damp strong paper towels, like Bounty, on hand to wipe the resin slurry you create with the pad; so you're not sanding the slurry back into the coverstock. A couple of swipes on each half should be good.
Keep the sanding pad wet while using circular motion.

If you want it rougher than 2000, I'd suggest the Light grey Scotch-Brite pad. That's 800 grit CAMI, which is approx. 1600 grit FEPA. (All Abralon and Siaair pads are FEPA grades.) Green SB pad is 600 grit CAMI or approx 1200 grit FEPA.