I've been trying to flatten the sole of a Stanley No. 80 cabinet scraper, using silicon carbide wet/dry paper on a granite tile. I've used this procedure several times before on planes and chisels, with no issues. On this project though, I seem to be getting nowhere fast.

I'm starting with 100 grit paper. The sole is just not getting flat. There's a bit of a hollow around the mouth opening and I want to get rid of that. I'm using a forward and back motion, but there are left-to-right scratches in the sole that are just not disappearing, in spite of the significant effort that I've put in so far. I'm not sure what is going on. Is the iron in these tools extra hard?