Here's a tip that worked for me when your files or rasps get rust on them or get too dull > Soak them over night or longer in muratic acid, as it removes the rust and takes the teeth down to new metal. I've used this method to clean ferrier's rasps and rasps I have found at garage sales that were so cakes with rust I thought they would never be usable. Just don't soak too long or you will destroy the file and soak in a baking soda bath afterwards as well use a good oil to seal the metal