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The Timesavers lapping compound is really nice. It does not embedd and the “sample kit” is rather inexpensive and would last us modellers a very long time.Cletus https://www.ws2coating.com/timesaverlappingcompounds/
I'm no expert but I think diamond between a valve & seat in-situ is not a good idea for the reasons we both stated - any amount diamond grit that 'sticks around' to either part, its probably bad news for running conditions. I suspect that's where the classic (aluminum oxide or maybe silicon carbide) lapping compounds can be used, because of the presumption that they will wash out cleanly.http://www.americanlap.com/Lapping%20Compound.htmWhat I was referring to in my post was a dedicated 45-deg lapping pot for the valve face (shown in the picture). The pot takes care of the embedded diamond issue. But that's different way of going about what I suspect for you is a 99.9% finished valve/seat combination that you are attempting to mate together as a fitted pair by lapping. I hear there are pros & cons, that's why I wanted you to see Terry's method & underlying logic.