I have a Nichilson BD 12" file that is all most 50 years old. It has been used to sharpen all kinds of things, bush axes, axes, lawn mower blades, etc over the years. Saturday went to use it to sharpen lopping shears, and it just wouldn't. So Saturday night and Sunday, it spent the day in a bath of white vinegar. Took it out this morning, and rinsed with water, then took torch and dried it off. Next sprayed it with WD-40 to prevent rust. Now it cuts like a new Black Diamond file! For more info, search "using acid to sharpen a file."