Prep Stone for Waterstones...
20mm x 20mm x 3" stick. Used to raise a slurry for sharpening on fine grit waterstones. The slurry is what does the cutting and polishing and the prep stone speeds up the slurry-making. After wetting the waterstone, rub it evenly with the prep stone which will create a slurry, even off the high spots and make for less frequent flattening for the waterstones.
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