The term USE A WHETSTONE ON can be summarized by two crossword solutions: SHARPEN (7 letters) and HONE (4 letters). Sharpening is the process of making an object, such as a knife, sharper by grinding it against a hard surface. HONE is a similar process, but it is used to refine the edge of a blade that has already been sharpened. Both processes involve using a whetstone, which is a flat stone made from a hard material such as quartz or diamond.