Atoma Diamond Plate #400
The Atoma Diamond plate #400 grit is the standard for whetstone flattening. It removes material extremely quickly and allows you to flatten your stones with relative ease. They are also great for major repairs as they can very quickly remove steel from a knife. We wouldn't make a habit of using it to sharpen knives though as doing so will pull diamonds off the surface making it less and less effective as a flattening or sharpening tool.