Rasps get dull, just like any other tool. Most people just pitch them when they get dull and buy a new one. But before you go tossing those olds dull rasps, or grinding them up into whatever, give sharpening them a try. No, I’m not talking about honing all the teeth with a microscopic stone. Even I’m not that nuts. To sharpen a rasp, we turn to chemistry. Blacksmiths have been using this trick for sharpening their files for decades. All it takes is a few inexpensive items from the hardware store, and some salad dressing.