As I've noted before, the acrylic primer on prepared canvases or available in stores is usually labelled "gesso." It's not actually gesso and shouldn't be labeled as such. For oil painting, I find real gesso to be a much better surface than acrylic primer. Egg tempera and tempera grassa should be used only with real gesso panels. Gesso should only be used on inflexible supports (i.e., panels), because it is too brittle for canvas and will crack.
Gessoing is easy and almost foolproof, but time-consuming. It takes an afternoon to gesso a panel. On the other hand, it takes an afternoon to gesso five, ten, or twenty panels, so it pays to produce them in volume. I generally invest three or four afternoons a year in making enough panels to provide me with a steady supply.
Here's how to make and apply gesso:
Hide glue (often labeled "rabbitskin glue" whether it contains any rabbit or not). Most major art suppliers have this.
Inert white pigment. This is powdered chalk or gypsum. The marble dust you can buy in art stores is chalk. Plaster of Paris is cooked (anhydrous) gypsum, but I have found it too gritty to make good gesso. (The word "gesso" means "gypsum" in Italian, since that's what Italians made gesso from. In Northern Europe, chalk was the traditional material). You can buy good-quality powdered gypsum from specialty suppliers like Kremer.
Titanium white pigment. This is optional. Some people like to substitute up to 20% of the intert white pigment in the recipe below with titanium white, for brightness. I haven't found it worth the bother.
Panel. There are various materials you can use for panel painting. One good option is to buy hardboard at the home improvement or hardware store. You can buy it cheaply in 4 foot by 8 foot sheets. Get tempered hardboard 1/4 inch thick. The staff at the store will probably cut it to size for you if you ask. Other materials you can use for panel include medium density fiberboard (MDF) and actual wood planks. Wood panels of any size, however, is best seasoned for 1-3 years, with planing to size if it warps, after it has been cut.
Wide flat brush. A good housepainting brush will do.
A double boiler. Or use one pan that can fit inside a larger pan.
Measuring spoons, mixing spoons.
Sandpaper. Several grits.
Preparing hide glue
Make the hide glue the day before you plan to gesso the panel. Hide glue normally comes in powder or granular form. Mix one part hide glue with 11 parts warm tap water. One cup makes about enough to size and gesso two 8 x 10" panels, depending on how many layers of gesso you apply. Stir the water/glue mixture for about five minutes, then let it sit for 6-24 hours or so. It will form a thick gelatin. If the weather is very hot (95 degrees farenheight+), it might not gel properly unless you put it in the refrigerator.
Preparing and sizing the panel
The edges of the panel should be smoothed with sandpaper or a rasp. Clean the panel with denatured alcohol to remove any trace of oil or other guck.
Now you want to coat the panel in a layer of hide glue. This is called sizing the panel because another word for hide glue is "size." You'll start by warming the glue to make it fluid. If you heat the glue too much, it will weaken the glue. As it turns out, hot tap water is about the right temperature to liquefy glue without damaging it. So fill the outer pan of your double boiler and put the glue into the inner pan. In about ten minutes, it will be about the consistency of milk (whole milk, not that low fat stuff). Brush the glue over the front, back, and sides of the panel. Give it a half hour to dry.
I generally add more layers of glue to the back. The reason is that the glue in the gesso on the front will be applying force to the panel. If the panel is large, this will noticeably warp the panel. So I generally add about four layers of glue to the back in order to counteract the warping effect that the gesso will apply to the front. This seems to help a lot.