I'll offer my experience and opinion about these "flattening stones" - don't know if it's applicable to your stone.
Originally Posted by Stuart Ablett
I bought one of the Norton flattening stones. After a while, I started having trouble with my water stones, even though I was flattening them fairly often. What happened is that the flattening stone was no longer flat.
On water stones, you always wear the middle the most (the stone becomes concave). When you use the flattening stone on your water (working) stone, you tend to wear the outside of the flattening stone more than the middle - so your flattening stone becomes convex. Soon, that flattening stone is just making your working stones concave (I guess maintaining the concave of the working stone).
Once I figured it out, I bought a DMT diamond plate and use that to flatten my working stones. No problems since.
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