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Dmt makes a 1200. But it isn't made for lapping. You can try it but it may ruin your plate. A piece of wet dry sandpaper is cheap enough and expendable.

Then you can buy fine wet dry sandpaper. I don't think that you need to smooth it more than 600. Get a piece of glass or a natural tile like marble or.granite. wet the sandpaper and the base which...

You got to lap the thing. I don't see how a flattening will kill the stone.as was said, get it flat with your atoma, then smooth it out with the slurry stone. And then see what you get. For coticules...