My First 318 Rebuild

Mainly the stems wear. OEM and premium valves were usually chrome plated. You can polish the stems and the non combustion chamber side of the valve with a drill press, wd-40, and 600 wet or dry sandpaper. There is thickness limit on the margin of the valve also. As long as the stems are within spec and you can get a good face with enough margin left, the valve should be good to go.