If you want to be hyper-traditional (and trust me, you don’t), check with Cennini, because you’re going to need to bury things, and run them in the clearest spring water and generally realize why, back in da day, there were lots of apprentices running around a workshop: to do the grunt work.
Grinding zinc white (a paint pigment, and not a very interesting one at that) is nobody’s idea of a party, and I have to admit, I skipped doing it… and my gesso would have been better if I’d broken out my mull (the paint one, not the food one) and grinding stone and ground away for 20-30 minutes. Still, it looks like my gesso will be pretty awesome once it’s sanded and rubbed (only another 30-45 minutes. Per panel.)
Not surprisingly, I don’t mix food pans and painting pans, mostly because... that would be dumb. I use a cheap, enormous speckled pan for my gesso, expensive isn't worth it the gesso, as its scrubbed off, starts to pull of bits of the pan (or, in this case, the enamel). Someday my pot will be like swiss cheese, but nobody’s eating out of it, so I don’t really care, unless it affects the quality of the gesso.
4-8 cups rubbing alcohol
2 cups rabbit skin glue
20 cups water
Approximately 10 cups each of precipitated chalk and zinc white (I tend to mix in several cups of marble dust in the 10 cups of chalk because I think that it tends to make a finer final surface).
1) brush all panels with rubbing alcohol, on both sides. You probably want the panels on drop cloths (not plastic; the wet panels will slide and the gesso will get smeared) or a surface you don’t care about. This will be messy. And difficult to clean up. You might consider wearing very old clothes; by the time this is over, you will be coated in glue-plaster and smell like a dried dead animal that has been rehydrated and cooked. And not in a good way. I’m pretty sure that Frederic Malle isn’t planning a perfume release based on that sort of scent.
2) In a large plastic or enameled (non-reactive. actually, who knows. Maybe you'd be fine with a highly reactive container. But it seems sensible not to test that.) container, mix your zinc white, precipitated chalk and marble powder or whiting together. Set aside.
3) Heat the glue and the water on a diffuser over the stove. Technically, you should do this on a double boiler, but seriously? Nuh-uh. Stir often, but DO NOT BOIL THE MIXTURE. You won’t be happy. Do you really want to boil glue on your stove? NO.
4) When the glue is dissolved, remove from heat, and, as you would put flour on top of pan juices to thicken a gravy or on butter to make a roux, dust some of the chalk/zinc mixture on top of the hot glue mixture and whisk in. Steadily add more, whisking in, and checking for lumps (some people prefer to pour the hot glue into the whiting, much as one would put egg in the flour for pasta. I already have a knack for scalding myself; I prefer to add the non-scalding material to the scalding material). Add more of the whiting mixture with each addition, until it is all in. The texture should be similar to skim milk, and will paint on with almost the appearance of milk.
5) Stir well, and let the gesso sit for 20-40 minutes in a warm-ish place (mine was outside in 50°F weather, so… I let it cool for less time). Although the gesso should be warm (it will start to jell and curdle as it cools), too warm will result in more air bubbles. However, when the gesso begins to cool, did I mention the curdling/jelling? It's problematic. If you happen to have a little plug-in stove that can be put where you're working, or are working in your kitchen, you may be able to warm the gesso ever-so-slightly and whisk like mad (reminder: your whisk should only be for art use) to try and get it all milky again. Even so, un-jelling/curdling might not happen. You need to work with a swift, steady hand, with even brush strokes in a single direction on each application. If you apply helter-skelter, you will understand why I say not to do it. But then it will be toooooooo late, and in order to get that panel back in working order, you, some sandpaper and a sanding block are going to spend some quality time together.
6) Paint each panel on one side, with brush strokes going the same way. Make sure to paint in order. When finished, turn the panels, and paint each panel on the other side (this will prevent warping, and allow for both sides to be used, which is particularly useful when you’re doing sketches).
7) On the second coat, paint with brush strokes that are vertical to the first coat’s horizontal, or horizontal to the first coat’s vertical. Do both sides. Again.
8) Do 3-5 coats, depending on how quickly the gesso cools. As it cools, it will be harder to paint on a smooth coat; it will become curdy. Better to skip the 4th coat than paint on an enormously lumpy coat, because it will just add time- and lots of it- to the sanding process.
9) let the panels sit for 48-96 hours (72 and more is better) in a dry place to cure.
10) Sand (I use 180 and then 220 sandpaper) in a well-ventilated space (it'll be messy) and wipe down before use.