Naniwa Flattening/Lapping Stone #220

A point worth repeating from the book: every whetstone wears unevenly over time, because you naturally use the same spot in the middle most often. Sharpeners affectionately call the result a "bathtub", a dip in the center of the stone. And with a warped surface, you can't hold a consistent angle anymore, no matter how good your technique is.

The Naniwa flattening stone at 220 grit is built to grind that dip back out. Honest note from the community: some users report that this flattening stone itself eventually loses its own flatness, and recommend checking it with the pencil-grid trick (draw pencil lines across the stone, keep flattening until every line is gone).

Good for: routine maintenance of any waterstone between 1000 and 12000 grit, whenever a dip starts to form.

Pros:

• A genuinely necessary tool for anyone serious about waterstones
• Works across a wide range of grits
• Compact size

Cons:

• Takes a bit of technique (the pencil trick) to verify you've actually gotten it flat

• Some users report it can lose flatness itself over time

• An extra tool beginners often don't expect when they just wanted "a stone"

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