Good advice from Ricky and I also would not use a sanding sealer on the veneer.
Wiping it down with metho will also allow you to see any glue spots the may have seeped through seems or just got smeared on the top.
If you have any glue spots, you'll need to thoroughly clean those off before applying any colour. Common practise is Goof-Off (near 100% acetone) and a small stiff plastic-bristled brush.

As for the sanding part, my preferred method is with synthetic sanding pads (aka: "synthetic steel wool"). They are available in varying coarseness, though unfortunately not in most hardware stores. FWIW the "green" Scotch-Brite pads from Coles or Woolies are roughly (no pun intended) about the equivalent of P400.

I've rattled on about synthetic sanding pads vs sand paper in numerous posts. Probably to the point of ad nauseam .