You can't sharpen a knife if your whetstone isn't flat. This diamond coated whetstone repair tool will help keep your whetstones in top condition.
Asides from repairing whetstones, this is also useful in knifemaking to remove high-spots from blades. If you don't have a disc sander this is an inexpensive way to flatten your blades.
Comfortably palm-sized, approximately 130mm x 40mm x 70mm in size. High quality industrial diamond coating.