While sandblasting, actually they usually use walnut hulls, is usually done by the pros for the best finishing I don't think it's an absolute requirement unless your gun has a lot of surface rust. That's just my opinion though and I'm new at this myself, so...
Here's a few leads.
4170 W 2100 S
Salt Lake City, UT 84120
I'm not having my FAL blasted but I am going to parkerize it myself. The parkerization process gives the steel a protected surface on its own, plus it provides a good surface for the DuraCoat to adhere to.
This is my first project like this so we will see how it goes. I'll post a few pics when I can get my hands on the wifes camera