Blade performance requires a clean surface which can be achieved by careful polishing with a simple wax polish.
WD 40 can damage the bonding as mentioned above. I have seen it used on metal blades to inhibit rust formation and potential icing.
A highly polished Enstrom blade will give 1/2 inch of extra power.
by reducing drag.
Tapping blades is a way of giving you the benefit of knowing all is well with the bonding on the intended flight.