The acrylic primer on prepared canvases or available in stores is usually labeled “gesso.” It’s not actually gesso and manufacturers shouldn’t call it that. 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 inert 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, are best seasoned for 1–3 years, with planing to size if it warps, after it has been cut to final size.
Wide flat brush. A good house painting brush will do. A double boiler. I don’t like commercial double boilers because there is too much contact between the metal pans. Instead, I use a pair of very cheap pans—one small, one large. I use an empty tuna can to support the small pan in the large 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 × 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 Fahrenheit+), 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 with hot tap water 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. h3. Making gesso Measure the volume of the remaining glue and pour it back into the double boiler. You will be adding 1.5 times this volume of chalk or gypsum to make gesso. Do this gradually, gently dropping each spoonful into the liquid to avoid making any bubbles. Distribute the chalk/gypsum around the pan so that it the glue soaks into it. Once all of the chalk/gypsum is in the pot, give it 10 minutes to soak. Now take a brush and gently stir the mixture, again trying to avoid making any bubbles.
Applying the first layer of gesso
For the first layer, spread it thinly over the surface of the panel, stroking back and forth in one direction. It’s not very opaque when wet. Let it dry; this takes 10–30 minutes, depending on humidity and temperature (dry days are best for gessoing panels). You’ll know it’s dry when it feels dry to the touch and any grayish areas have disappeared. Cennino, a 15th century Italian artist and writer, suggested rubbing the first layer in with your hand rather than spreading with a brush. That’s messy, but works just fine and may improve adhesion. If the gesso in your pan is getting thick, it means that it’s cooling off. Replace the water in the double boiler with new hot tap water. Don’t overdo it; this is usually necessary only once every 30–60 minutes or so.
Applying the rest of the gesso
You will apply 6–8 layers of gesso. Brush strokes in each layer should be applied at right angles to those of the previous layer. Each layer is best applied shortly after the previous layer has become dry. It’s best to apply all layers in one day, so that they will bond with each other. If you get cracking, that means that you’re applying the gesso before the previous layer has dried. More layers will fix this. If you get little pits in the gesso, then you’re painting with gesso that has bubbles in it. Let the gesso stand for a half hour before applying any more, then rub the next layer in with your hand. Once you’ve applied all the gesso, let the panel dry for at least three days. You can clean the brush, pan, and anything else that got gesso on it in warm water.
Smoothing the panel
Start by using a metal file to chamfer all of the edges of the gesso, so that they are at a beveled angle inward. This protects against cracking, should the panel strike something (I’ve had this happen with a large panel that I put a lot of work into, and it’s very irritating). To get the panel smooth, I like to use a sanding block, starting with 400 grit sandpaper and moving to finer grits at the end. This produces a beautiful, eggshell-smooth finish that is almost too beautiful to paint on. If I’m going to be painting with oil, I like to apply a final layer of hide glue to the smoothed surface of the panel. Without that, the gesso is too absorbent. Others use a thin layer of shellac or varnish to reduce absorbency; I haven’t tried that. For egg tempera or tempera grassa, plain gesso works great.
Update 24 May 2014: Bolded the sentence that says “Gesso should only be used on inflexible supports (i.e., panels), because it is too brittle for canvas and will crack.” We’ve had a couple of questions in the comments from people who have applied gesso to stretched canvas. Gesso is for rigid surfaces such as panels. Don’t apply it to fabric unless that fabric has been glued to a panel first.