How to flatten sharpening stones
Over time, sharpening stones wear more in the middle than along the edges. If you continue to sharpen in the dished area of a stone, your tools' cutting edges will have slightly rounded edges. That's why it's critical to remove the high sides.
Waterstones and oilstones wear at greatly different rates: The softer of the two, waterstones, dish out faster and need to be flattened more often. We recommend flattening them at the beginning of every sharpening session-it's a 15-second job in most cases. Flatten oilstones about every 10 sessions. Another benefit of flattening your stones: You'll remove embedded metal filings that impair the stone's sharpening action.
Flat is where it's at
An easy, inexpensive option for flattening stones is to use sandpaper, as shown below. Start with a reliably flat surface, such as a piece of 1⁄2 " thick glass (with edges ground smooth). Apply 100-grit, self-adhesive, wet-dry sandpaper and rub the wetted (or oiled) stone over it until flat. Repeat with a few strokes on 400-grit paper to remove any coarse scratches. If you flatten your waterstones every time you sharpen and these stones show little wear, skip the 100-grit step.
A flattening plate —a harder, abrasive stone, shown— also makes quick work of dished-out stones. If your sharpening stones did not come with a flattening plate, you can buy an effective silicon-carbide version. To flatten a stone, wet it with water or oil (depending on the type of stone) and rub the flattening plate evenly over its entire surface until all grayish evidence of previous sharpenings has disappeared. If you can't tell if it's flat, simply make a series of pencil marks across the stone, and then flatten until the marks vanish.
Finally, you must occasionally flatten your flattening plate as well. Use the sandpaper method described on slide 2. Make pencil marks down its length and rub until gone.
For convenience and portability, we like Norton's three-waterstone kit, shown, because two stones soak while you sharpen with the one sitting on top.