The following is excerpted from “Make a Joint Stool from a Tree,” by Jennie Alexander and Peter Follansbee.
Like with any tool, there are a lot of different ways to sharpen piercer bits. Files, stones, burnishers and more. We sharpen them on the inside only. Many different methods will work, including using burnishers, files and stones. The best tool we have found for sharpening these bits is a triangular burnisher. If you can’t find one, then you can take a worn-out triangular file, grind off its teeth and mount it in a handle. Then you can use it as a burnisher to turn the piercer’s edge from the inside.
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Mark Atchison, a blacksmith we have worked with for years, has a nice method of getting these bits really sharp. He uses a worn-out round file, and grinds the end of it square and uses it as a burnisher to run down the inside edge of the piercer. Save your old worn-out round files; you can use various-sized burnishers to fit different-sized piercer bits.