My belt sander thinks it's a Van de Graaff generator

Either the belt's not at ground or your hands not at ground. Try connecting a wire from your hand to the frame (might want to use a resistor to limit current) and see if the sparks are stopped. If you are grounded and the machine is grounded it is the electrical equivalent of a wire connecting you to ground, the same potential (should be) as power plug ground.

RogerN

I remember hearing something about set the power tool down on the ground, then let go, then pick it back up and begin using it. This will result in the tool being grounded right along with your, and the plug's ground potential.