I'm using leather punches to cut polypropylene film, but the round one isn't sharp enough to do a nice job. The film cuts nicely with a scalpel or scissors but I need 5mm discs (with a 3x2mm hole in them, cut using the rectangular punch, or a more complex cutout using a scalpel).
The rectangular one also needed sharpening, but this was easy -- a small diamond file and an approach like sharpening a chisel.
But I just can't get a good edge on the round one. I don't know if I'm having trouble holding it at a consistent angle, or what the problem is. I I've got sharpening stones, a diamond file, and various abrasive sheets.
Is there a trick to sharpening small, hollow, round, steel things like this?