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I am a bit of a rock hound and some of the hones I have made are ver hard. None of them have killed my DMT's, but they do require to be cleaned on occasion. They look dead but spring back with a little soft scrub and a nylon brush. I lap under water as opposed to under running water. It is quite effective. Give it a try. Do 20 figure 8's and see where you are at. I'm fairly certain if it is hazardous to your plate you will feel it right away.
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I have done a lot of lapping both ways and find that I get more of a swarf accumulation when I hone under water as opposed to under a steady stream of water; but both ways work.
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I have lapped multiple old oil stones starting on a flat piece of concrete or bricks. It works fast. After that I'll lap them on w&d paper