You need to use sharp paper. One thing that gets amateurs is not changing their paper enough when sanding. Beginners will (without even realizing it) start pressing harder on the block to compensate for their paper going dull. Pressing hard while sanding creates waves. When I do body work (I'm a professional) I end up with a pile of used paper on the floor. You also need to use the correct grit for what you are trying to accomplish. Have a supply of 40, 80, (120 I use this many people don't), and 150 or 180 for sanding your body filler.
