David, there are many reasons for making things you can buy, from understanding how things work, building skills, learning about the history of toolmaking, learning associated processes amd material properties, taking on a challenge, and possibl producing something better for your needs than what you can buy. Rasps, by the way, aren't always cheap. Certainly not handmade custom shaped types.
Smon, my understanding is that the "sharpening" of new files by sandblasting is a sharpening step, not necessarily what we mean more generally as "sharpening", and that particular step is the removal of the wire edge after forming the actual point. A strop is used for the same thing on straight razors and similar processes on other edged tools. The strop doesn't actually sharpen by refining a point using an abrasive -- it's just leather and the blade is run backwards over it, it breaks of a tiny bit of excess past the end of the tip. I believe that's what the sandblasting does.
