Peter Breu replies: An acid dip works extremely well. It is a mild phosphoric acid and is very easy. I use this periodically on all my files and even very worn files end up like new. We often overlook dull files even though we sharpen everything else in the shop. I buy it from www.moonssawandtool.com for $18.95 a quart (#1QSP). A quick tip—pour it into a 1-1/2″ PVC pipe that is capped off on an end to reach the full length of longer files. I am particularly pleased with the results on my rasps—just like new! The best tip for caring for files is careful storage—don’t let them sit on top of each other!