Well, after a week or so of diligently scraping down and smoothing the gessoed wood panels, in accordance to the traditional method, I was dismayed at making such slow progress.
The test panel developed a small crack in what I had planned as the top of the painting, almost like the crack that the real painting has that goes from the edge of the painting to the top of Lisa's head. I thought that I might as well try a non-traditional method of smoothing the gesso layer on that panel, and stocked up on various grades of sandpaper.
This morning, I got out my trusty palm sander, plugged it in and sanded away. I used 100 grit to smooth the gesso, and 220 and 600 to finish. It's BEAUTIFUL. Not without flaws, but one can see why the old masters used this surface for painting. It's snow white and smooth as glass. I had thought about doing a few passes over the panel with 1000 grit paper, but it's not supposed to be too shiny. The layer has small pinholes in it from the gesso being stirred too vigorously and standing on the heat too long, but I'm hoping they won't be noticeable in the final painting. And this is the test panel, after all. When I redo the gesso on the hero panel, I'll be more aware of those issues. I also decided that I'll invert the panel and put the crack at the bottom in the darker areas of the painting.
The hero panel has such severe cracking at the joint between the panel and the frame that it'll have to be stripped and redone. It's got damp rags on the surface now, and I should be able to scape off the gesso this evening. I've ordered new supplies and will re gesso that panel once things arrive.