From The Practice of Tempera Painting:
"Before beginning to scrape the gesso, powder some charcoal, and sprinkle it over the gessoed surface. Wipe it off with a cloth, and the gesso will be colored gray by it. Where you scrape gesso away, it will show white, and by this simple means you can tell what parts need more scraping as you work.
"Take the scraper in both hands, with the burred edge toward you, your thumbs planted firmly in the middle of that side, and the ground edge away from you. Hold the blade vertically, and draw it toward you. If the edge has a good burr, it will scrape a rectangular shape of white in the gray ground. Do not try to take off too much at each stroke. Low spots in the gesso will show as gray surrounded by white, and you must work them down gradually. The way to do this is to work around them with a crisscross, herringbone motion. Do not work over one place too long, but move on and come back to it."
Here's a board where I tried scraping. I picked up the scraper from Natural Pigments some time ago and figured I'd try it in place of sandpaper. I did manage to smooth out the surface considerably but I still felt the need for some 600 grit sanding. I didn't do the charcoal thing as this was simply a little test. If I do this properly, there's supposed to be no need for sanding.
More boards are on the way from Realgesso.com--another half dozen 9x12" and a couple of 11x14".
Thompson, Daniel V. Jr. The Practice of Tempera Painting. New York. Dover Publications. 1962. (Yale University Press. 1936.) pp. 30-1.