The surface is fairly flat. Other reviews point out that the stones are a few millimeters smaller than the slots they sit in and therefore the box cannot be used for actual sharpening. I don't think it's a problem, I put a small amount of putty on the top of each and now they're set like a rock. A folded piece of paper tucked in there would probably work in a pinch. the duration for which they will last. I started using them to reshape a knife with a broken tip, by the end the roughest stone was almost entirely gone. Its surface is now thinner than the ultra-fine stone included in the kit. Waste of money unless you really need to sharpen occasionally (even then a better double sided stone may serve you better).
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