How to Sharpen Metal Files
Channel: Build Something Cool
Date: 2016-04-17

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=seMDAAAAMBAJ&pg=PA64&lpg=PA64&dq=Chemical+file+sharpening&source=bl&ots=Bse9zY6U2B&sig=_zpkvcVtRj51XcbcOImlNjJ-y3w&hl=en&sa=X&ved=0ahUKEwibldXZ-YrQAhXBFZQKHW8cBGwQ6AEIPDAE#v=onepage&q=Chemical%20file%20sharpening&f=false

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=ApFJAAAAIAAJ&dq=Workshop+Receipts%3A+For+Manufacturers+and+Scientific+Amateurs%2C+Volume+2%EF%BB%BF&q=235#v=onepage&q=235&f=false