Grinding paste, also called valve lapping compound, is an abrasive paste used to seat engine valves against their seats and for other precision fitting work.

It is normally supplied in coarse and fine grades in one container. The coarse grade cuts the initial seat where there is pitting or wear to remove, and the fine grade follows to produce the smooth, continuous contact band that actually seals. Working straight to fine on a pitted seat takes forever, while stopping at coarse leaves a surface that will not seal properly.

Technique is simple but the cleanup is critical. Lap with a light oscillating action rather than continuous rotation, check for an even matt ring around the full circumference, then remove every last trace of paste from the head and ports. Abrasive left behind will wreck the engine.