I was grinding something tonight and the wheel on the 4 1/2 grinder was not true, it was bouncing around like crazy.. I didn't think I could true a wheel, but I took my diamond dresser for a bench grinder and trued the fiber impregnated wheel. It worked great.. I didn't think the impregnated wheels could be trued, but it was worth a try since I didn't have a new wheel available.
Hope that helps someone.