Sharping file is fast and easy. Watch this video and learn how chemically sharpen your metal files.

Give it a try, it really dose sharpen the file. I promise you, you will be surprised. One side note the file must be dull not damaged to work. :-)

Popular mechanics article from 1992 with microscope before and after photos
https://books.google.com/books?id=seM...

Check out his link on Google books to learn how it works.
Workshop Receipts: For Manufacturers and Scientific Amateurs, Volume 2 page 235

https://books.google.com/books?id=ApF...