There are two things to consider in balancing a prop; mass balance and
aerodynamic balance. You can static balance a prop, but if the mass
distribution is not symmetrical, there will always be a V^2/r
unbalance. If the blades are not symmetrical in chord and pitch at each
station, there will be a force unbalance that will cause a whirl of the
engine on its mounts. This cannot be taken out with mass additions.