Gesso
Pronounced with a soft g like gypsy or George. From the Italian for gypsum, a major component.
This thick white liquid is primarily used as a ground for painting but can also be used to build up areas for carving on frames and is used underneath gilding. It can be coloured. Gesso for gilding is often coloured red. You can buy ready-made black “acrylic gesso”.

Gesso is made with calcium carbonate (also called whiting, chalk and gypsum) in a binder. It is painted on the canvas, paper or wood panel surface to create a ground on which to paint. Sometimes white pigment (usually titanium, sometimes zinc) is added to make the gesso very white.

Genuine gesso (also called true gesso) uses animal skin glue (hide glue or rabbit skin glue also called “size”) as the binder and the artist often makes the gesso him/herself, using a double boiler to melt the glue powder and adding the whiting. Rabbit skin glue is now also available ready made and just needs to be warmed.
One recipe for traditional gesso: 3 parts size, 1 part chalk (whiting), 1 part pigment powder. It is a rather lengthy, messy, smelly process of soaking, heating in a double boiler and mixing.
“Acrylic gesso” is more correctly called “acrylic primer” and should not really be called gesso. It uses an acrylic polymer as the binder for the chalky powder. It is made up of upwards of 14 ingredients. You can also buy ready-made black acrylic primer.

Genuine gesso is less flexible than the “acrylic gesso” and is usually painted on a non-flexible surface such as a wood panel rather than on stretched canvas, so that it will not crack. For paints that need an especially porous surface, like egg tempera, genuine gesso is usually preferred to the acrylic gesso/primer.

The acrylic primer varies a lot in quality and poor quality products can provide a less absorbent ground than is often preferred. Good quality acrylic primer is a very good product for oil painting and acrylic painting. It does both steps of the surface preparation in one- it both sizes (seals) the surface and gives a ground for painting. It can also vary in absorbency, with some products called “acrylic gesso” rather than “acrylic primer” being more absorbent and chalky and particularly suited to applications which require an absorbent surface.

Roberson's Acrylic Primer

Acrylic primer differs in thickness, opacity and grittiness of surface texture, depending on the manufacturer. It is usually too thick to use straight out of the bucket and should be diluted with water until it is the consistency of heavy cream. Most primers have instructions that advise you apply three thin coats rather than one thick coat. A very thick coat may crack as it dries. The first coat is often scrubbed into the weave of the raw canvas in circular motions to be sure that it is well sealed. The first coat will soak into the canvas or panel and act as its own sizing (sealer). Then subsequent coats are applied in alternating directions across the canvas. To get a very smooth surface you may wish to sand with sandpaper between coats. Some acrylic gessos are designed to have a harder surface specifically so they may be sanded smooth, but as they are less flexible they may crack on a movable surface such as stretched canvas, so should only be used on rigid surfaces.

For oil painting it is especially important that the oil never reaches the substrate as it will rot the canvas, paper or wood. Traditionally oil painters seal the surface with rabbit skin glue and then prime the surface with gesso (glue with chalk). Using these two layers assures that none of the oil will seep through. Some artists who use ready-made stretched canvases will apply an additional layer of acrylic primer to the surface to ensure that it is well sealed.

For painting on paper you may wish to prime both sides of the paper (one after the other dries) as the paper will curl when it is wetted by the primer. Painting the other side then un-curls it. For oil paint on paper you may want at least three coats.

Priming your painting surface is part of properly creating a painting. The underlying structure is very important to the longevity of the painting as well as to the appearance. Primer creates a surface that is sealed just enough to prevent the paint seeping through to the substrate (canvas, paper, wood), but is absorbent enough to hold onto the paint. If you were to paint on an unusual surface like a rubber toy, the paint might not adhere properly. But if you prime the surface with acrylic gesso/primer first, then your paint will go on properly and stay on. The primer is stickier than paint and will glue the chalk to your substrate and create a better surface to paint on.

While the gesso/primer is wet it may leach colour up from the substrate and cause discoloration to the whiteness of the gesso. The glues in plywood, the resins in wood panels and in stretcher bars may be water-extractable. Sealing the wood or canvas first with a sealant medium such as Golden Acrylic’s GAC 100 will prevent Support Induced Discoloration (SID). Sealing (sizing) with rabbit skin glue does the same thing if you are using genuine gesso. Then prime as normal.

Some artists prefer that the substrate shows through underneath the paint and so they use a clear primer. This is usually an acrylic matte medium. This is a thick white liquid that dries clear so you can see the canvas. The texture is very different to gesso since it does not have the chalk powder in it, the surface is smooth and not as absorbent.

Be warned that priming can be a messy business. Gesso/acrylic primer dries quickly on brushes and can stain clothes. Be sure to use drop cloths and wash everything as soon as possible.

Many artists use the word gesso as a verb meaning “to prime” as in “I will be spending the day gessoing canvases in the studio”.

Some artists mix gesso in with their paint as a painting material.