Picked up a tip off the internet, and thought I would share with anyone that trims hooves. Cover the dull rasp with muriatic acid for about 4-6 hours, take out and wash with soap and water or neutralize with baking soda, oil, and use. I had 3 very dull rasps that I did last night, and tried one out today. Cuts like new. Saved $75+ with $6 of muriatic acid, and returned the acid to the container for future use. Of course this could reduce the number of old rasps available for knife making.