I think I'm going to glue my Naniwa SP 8k to a piece of marble tile to keep it flatter. Checking the stone through a honing session with a straight edge and as it is drying shows it moving around quite a bit through what I suspect is uneven water absorption. I don't soak my stones, just spray and go. I think the thinness of the SP stones contributes to the warping. It's easy enough to keep them flat by lapping but I attribute 90% of the wear on the stone to this continuous lapping.