I agree grinding is the way to go.
We have the same problem if we order commercial metal stamps. The shoulder cuts into the metal (it has more to do with the springiness of the material, not just how you hold the stamp), and modeling tools won't do anything to smooth that out. So all new stamps have the shoulders ground off before we even start.
Our letter stamps are zinc die cast stuff, which can corrode a bit but not really rust. Somewhere I have the instructions for chrome plating tools, if anyone is really interested. However, I find frequent use is the best corrosion prevension in our workshop, even for the very corrodable high carbon tool steel we make into our own stamps.
Leaving the backs completely alone for aligning purposes, I did grind off the shoulders and face corners. No edges except on the letter shape. I used a flex-shaft tool and Mizzy heatless grinding wheels. Easier to control what is removed that way, and it's fast on die-cast. A few got a good polish after that, but the rest did not, as it did not really seem to matter. I would use a file before a belt sander, just so I could see what was going on.