Go after the noise at source - often a better approach than to try and clean it up when it gets to your board. Sounds to me like the hand sander has arcing brushes. Try a 47 ohm/0.1 uF suppressor capacitor across the active to neutral inside the sander case if possible. Suppression capacitors have a special winding of the capacitor to make it lossy and they work a treat on most noisy brushes.

HTH
BrianT