Gotta love those Greenlee chassis punches

I've always felt they were very high-quality punches. I've got quite a few of them, but they're old, and I used to use them to punch holes in steel car bumpers(!) to mount CB antennas and trailer electrical connectors, so unfortunately most of them have become dull.

I'm wondering if a DIY'er can sharpen them (and if so, how do you do it?), or should I just buy new ones. Thanks, everybody.