As Skeet has mentioned use Isopon P38, other products are ok some are better but it's hit and miss.
Isopon is the genuine article but a tip for using it is to make sure you use wet and dry sandpaper but make sure that you use it "WET" and keep washing of the body and paper clear of all debris, this will give a much smoother surface and if you slowly go finer and finer on the paper grades then you'll end up with superb results.
So remember children the moral of this story is "Keep it Moist"