My previous post did not describe the process very well, so here are some photos.
On the left is a new nail, on the right is one that has its head hammered flat.
This file has solder in the grooves. I am pushing the nail in the same direction as the grooves.
The process forms tiny teeth in the end of the nail.
The process becomes more effective, the more that you scrape. You get more and better defined teeth on the nail. Also works just as well on aluminum, glue, etc.