The Atoma Diamond plate #400 grit is the gold standard for whetstone flattening. It removes material extremely quickly and allows you to flatten your stones with relative ease. While other synthetic truing stones will abrade and wear over time they will tend to lose their flatness especially quickly when truing exceptionally wonky stones. The Atoma in comparison is mounted on a solid aluminum block that will remain flat through all the highs and lows you throw at it!
They are also great for 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. Should you wear out your diamond plate, you can peel off the diamond pad and replace it down the road when that time comes.
While it will cut a bit slower than the #140, it will leave a smoother finish on the stone as is what we usually use to flatten our higher grit stones of around #5000 and up or to kick up a quick slurry on a natural stone.
