The light rheostat is not enough amperage to use with a buffer,
there are other controls available in the heavy duty wiring section
would go to radio shack or the likes and try for at least 30 amps.
Also, keep in mind that most buffing motors are brushless induction
motors. that type of motor generally wants to run at a set speed
determined by the way the motor is wired. they're not usually well
suited to speed controls of any sort. If the motor has brushes and a
commutator, then it's controllable.