I've finally decided it's time to bite the bullet and become a painter who preps my own supports.
"Support" is the general term for what an oil painting is painted onto, either a wooden panel or stretched canvas. I prefer wood panels to stretched canvas because the surface is smoother and more rigid.
Previously I have mainly used factory-gessoed wood panels, like GessoBoard. Gesso is the chalky white paint that is layered on a canvas or wood panel before you start an oil painting. But I've decided that if I'm going to spend 60 hours on a painting, I may as well spend a couple hours preparing the surface.
So being an all-or-nothing type, I dove in and spent 3 solid days layering 19 panels with 2 layers of rabbit skin glue and 5-6 coats of homemade gesso, sanding between each layer. My right deltoid muscle aches to say the least.
I used traditional gesso materials from Sinopia (glue crystals, chalk, and white pigment) and combined them using their traditional gesso recipe. For the wood panels I used ArtBoards of all sizes, rectangles and squares from 6 x 6 inches up to 18 x 24 inches.

First I soaked the rabbit skin glue crystals in water overnight, which made a transparent, gelatenous gray lumpy mixture. Then I put the mixture into my double boiler, and when it warmed up it got clearer, runnier, and became a thin, watery glue. It spread really easily onto my wood panels with a housepainting brush, but immediately began to sink in to the wood and dried almost instantly.
Re-reading the directions, I found out I had to do TWO coats of the glue. It wasn't too horrible, and I have a good ventilation system in my studio, but there was definitely a distinctly funky odor. I don't know exactly how they make rabbit skin glue, but I imagine vegetarian painters don't use it.
Once the panels were sealed with the glue, I mixed together the chalk, white pigment, and remaining glue mixture and warmed it to make the gesso. It made a watery, not very paint-like liquid, so I had to play around with the proportions a bit. But I found it works best if it's slightly more watery than housepaint, so you can paint thin layers.

The sanding was the most tedious part, and my arm began to really ache. The second and third day I tried to use my left hand as much as my right to sand, so now both my arm ache.
Anyway, the project was a success - I now have 19 beautiful panels with a silky/chalky/smooth surface, plus a dozen teeny tiny panels that were just laying around the studio, for little oil sketches. Hopefully I won't have to do this again for a long time!