Eric is tellin ya right ! I use a small toothbrush type brass brush too. I worked as an aerospace machinist for 12 yrs, used all types of files for those 12 yrs. Not saying I am a expert, just got plenty of callouses from pushing files for a few years.
Keep your file in the best shape by keeping it clean, and rust free. Never use any pressure on the backstroke, or better yet, pick the file up on the backstroke so it does not touch the work. File card bristles are a little too big in diameter to clean some of the smaller finer teeth of smaller files, so the brass brush works better. Try not to touch the teeth of the file with your sweaty hands, will rust it quick. If you are going to store your file for a while, especially in the humid south give it a light coat of WD-40. Next time you get ready to use it, just clean with a little acetone or alcohol, chalk it up and make some filings ! Repeat the WD-40 when finished for the day.
I also keep a 12-14" mill bastard file for Broadheads ONLY in my archery tackle box. I mark the file with a sharpie, side 1 & 2. Side 1 is for hogging down new Wensel Woodsmans till I get all three sides cleaned up and almost shaving sharp. I then flip the file and use side 2 for the finish work. A quick strop on wooden block covered in thick leather and ready for surgery.
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