I had to google Carborundum stone, only ever known them as whetstones! That's exactly the plan, I've got an older carborundum stone around here that I use for odds and ends so I don't ruin my good waterstones. I'll hand grind with the teeth of the file so I flatten them before sanding away, so they don't bite in too much. That will work on the single-cut, I'll just have to go easy on the bastard file at first and see how it goes. Cheers for the tips about finishing to a polish, I probably would have left things too rough.

One day I'll likely purchase a good set of purpose made tools.

In other news my super cheap fret erasers from Amazon turned up, and they appear to be the real deal and no issues, so that will make life easier.