In the days back when I was an apprentice fifty plus it was my job to re sharpen Magneto and pivot files. Battery strength Sulphuric Acid was used, usually left in the acid for about four hours. Also there where a number of companies at that time who would re furbish files, they did a good job of it to, I still have a couple though they would only refurbish branded files like "Stubs"& other top makers.

The only lessons in my life I truly did learn from where the ones I paid for!