Here is the figure of the Virgin: I'd show her hands too, but for some reason my scan is a bit blurry. Not enough pixels.
OK, here is the first stage of the underpainting on the actual panel: I took the final sketch and transferred it to the panel with graphite, cleaned it up, made minor changes, and sprayed it with a bit of fixitive. Then I went over the drawing with peach black oil paint, thinned with mineral spirits to the consistency of ink. When this dried throughly, I applied an imprimatura: a transparent wash of oil color over the entire panel. I thought Durer and Grunewald used a warmish ochre color; but Joseph Shepard's book How To Paint Like the Old Masters said to use burnt umber to create an imprimature like Durer would have used. I think I compromised and used burnt sienna mixed with a bit of umber but if I could do it over, I think I would use a red ochre.
The dark area behind the head of St. John was a place where the gesso was too absorbent and took up too much of the imprimatura.
After I applied the imprimatura, and it dried, I worked up the figures in a grisaille: I used black and white paint to make a complete "grayscale" of the figures. You can see the completed grisaille of St. John on the right of the panel. After all the figures were painted in grisaille, I then began to glaze color over them.
I really should have made more photos as I worked, but I got into the painting and forgot. This weekend I'll photograph the finished painting and show here. I'll also say a lot more about how I worked and other sources for the painting.