As I understand, valve lapping is done to ensure a seal between the valve and its seal ring at the end of the shaft. I am just curious how valve lapping compound facilitates that? Does it make a chemical reaction with the metals on both ends to help mold them in shape?
Valve lapping compound is an abrasive. As you rotate the valve and apply pressure towards the seat, you are wearing both valve contact face and valve seat at the same time. This action makes both surfaces match each other exactly, which when valve spring pressure is applied, allows the valve to completely seal with the seat.