planalp, you always type faster than I can read so having trouble keeping up. So, it's screwdriver into mouth, then into Comet, right?
Good back up, hadn't thought of using any of the scouring powders so will have to check as don't always have lapping compound. Is Comet a soft material such as limestone? Never thought about checking that but we used to put some Bon Ami down the carb on engines which had been babied after a ring job. Some times the abrasive seemed to allow the rings to seat. As it would have to come apart otherwise, it seemed a worth while attempt.
Diatomateous earth (floor dry and such) might work also, if Comet is hard enough.
I'd think Comet tastes better than grease based lapping compound although I suppose one could mix some fine carborundum powder with a good extra virgin olive oil?
Wasn't it here some years ago that someone posted to an oil thread, "What's the deal with extra virgin olive oil." Post came back, quick as a flash, "The more virgin the oil, the uglier the olive. " Still the best line ever.
Good tips. We used to deal with a lot of screws and always kept an oil sample bottle full of Comet. Stick the tip of the screwdriver in your mouth, dip the tip of it in the Comet then engage the screw, spit Comet residue out of your mouth about every 5th screw or so. I recall my bowels being more regular back then......
So, if you don't have any valve grinding compound around but do have some Comet it will do in a pinch, but you have to get the tip of the screwdriver wet first so the Comet will stick to it.