Which product is the best to use to flatten my Japanese water stones?

DMT has engineered a product designed specifically for this purpose; the DMT Dia-Flat® Lapping Plate. Available in 2 grit sizes: 95 micron (160 mesh) for finer grit stones or 120 micron (120 mesh) for 1000 grit and coarser stones.

Both are designed to quickly flatten waterstones or any other type of sharpening stone. The Dia-Flat® is able withstand the extreme service conditions incurred when flattening other abrasive sharpening stones due to the proprietary DMT® Diamond Hardcoat™ process.

Use under running water or submersed in a pan of water for best results in order to quickly flush away the slurry that is generated when flattening. Abrade the surface to be flattened across the Dia-Flat® surface using long overlapping strokes in a figure “8” motion and light to moderate force – be sure to use the entire Dia-Flat® surface. A figure “8” motion promotes even, consistent wear of the stone being flattened and the Dia-Flat® Lapping Plate. The figure “8” motion and finishing with lighter pressure will reduce any scratches from the flattening process. (For complete instructions refer to below link.)

IMPORTANT: Do not use in a sink that has a garbage disposal unit as the slurry is detrimental to the disposal bearings.

Always clean/dry your DMT Dia-Flat® after each use as build up of material on the stone from flattening may be significant.

Use and care: https://www.dmtsharp.com/resources/written-sharpening-instructions/

Diaflat Instructions.            DiaFlat Video

Some DMT Diamond Sharpeners could be used (and have been used) for flattening, however, they were designed for sharpening, not flattening, therefore, DMT assumes no liability for sharpening stones worn out due to this method of use.