ORIGINAL: Flyboy Dave

Here is a nice tutorial on valve regrinding and re-seating:
http://home.hiwaay.net/~langford/corvair/valvejob.html

Notice the color on the valve where the lapping has taken place.
It is grey. This is what happens when you lap a valve, no matter
what grit or compound you use. The grey part shows the rebuilder
the parts of the valve and seat that are making contact. The mechanic
wants to see 100% contact between these two mating surfaces.

If there is an area that is still shiny, and not sealing, the mechanic
will continue to lap the valve until the 100% contact and sealing of
that valve has been accomplished. In this tutorial the mechanic is using
blueing as a visual aid on the valve seat. If the bluing is not removed
100% by the compound, the valve is not fully seating and sealing.

Every engine rebuilder knows these procedures well. Not every engine
rebuilder actually does the valve job. Most heads are sent out to a specialty
automotive machine shop for this work.

In our application, using polishing compound as a paste, we are
simply cleaning the sealing surfaces. This allows us to see the
area of contact and make for a perfect sealing of the two parts.

I have used this procedure of cleaning and inspecting valve components
successfully since 1970 as a professional, factory trained engine rebuilder.
I have used this procedure for 20 years on my four stroke aero-model engines.

Trust me, you cannot hurt a valve or a valve seat by cleaning the surfaces
with polishing compound.

FBD.