+1. :yes: The file cards have one side with wire bristles. They are on an angle on the face of the card. To use it properly, you draw the card against the angle wires. If you push it, it doesn't have the tenacity to get into the grooves of the file.
I was reluctant to offer my method for immersion sharpening. I use muratic acid (pool acid), which is hydrochloric acid. It's some nasty stuff...believe me. You need gauntlet type chemical resistant gloves, face/eye protection, and long pants and shoes. Actually wear clothes that you don't mind trashing if need be.
Do this in a well ventilated area, and don't spill a drop. I use a plastic mud tray for drywall, and submerge the file, and eventually flip it. It doesn't take that long. I'm thinking about an hour or so. The file can be neutralized with a clear water bath.
WARNING: This method is extremely dangerous. Do not attempt if you have any apprenhensions. The cost of a new file is much less than the repercussions of getting blinded, or getting acid burns treated. Disposal requires a local depository or service that handles toxic/hazardous waste.