Sometimes art terms can be confusing – some words mean more than one thing, or one product can be called two or three different names. A case in point is the words that we use for the materials and methods used to prepare surfaces for painting.
Although it is possible to paint in oil or acrylic on an unprimed support, you will find that there are some difficulties with paint handling if you paint directly on raw canvas, paper or wood. With acrylic paint, canvas can repel water so the paint beads up and doesn’t flow. With oil paint, canvas can soak up oil and produce halos of it around painted areas, or leave the oil paint crumbly and under-bound. With either paint, the weave could be more visible or bumpy than you want, or the colour of linen may be too dark; paper can absorb oil or buckle from water because it is too lightweight; wood can absorb paint unevenly in the stripes of the grain or absorption of water can cause it to swell or split. So there are treatments for these surfaces that are commonly used to make them easier to paint on. By using size to seal the surface followed by a primer, gesso or ground, an artist can create a surface that allows easier, more controlled painting. A second consideration is that these treatments also help create a more stable surface so the painting remains unchanged for longer. For permanent paintings you should be as concerned with the proper preparation of the foundation layers of the painting that are perhaps not visible (the support, the size and the ground) as with the layers that you do see (the paint, mediums and varnish).
Size, Primer, Gesso and Ground
Size, primer, gesso and ground are terms for the parts of the surface of a painting between the support – canvas, wood or paper – and the paint. Sizing is pretty clear – it is the first step of sealing the support. But there is some confusion about the other three terms – primer, gesso and ground – which are often used interchangeably, though not always correctly. Materials manufacturers are not consistent with naming, which can add to the confusion. Though some of these terms also apply to surfaces for works in dry media like pencil and pastel, and to other painting media like watercolour, egg tempera and encaustic, I am mostly focussing on how they are used in oil and acrylic painting because there is a wider range of choices available, so there is a bit more confusion. And the other media can be extrapolated from this information.
Sizing agents include: acrylic size, acrylic polymer used with or without acrylic fabric stiffening medium, acrylic matt medium, PVA size, and rabbit skin glue.
A size is a glue that seals the surface to reduce absorption. The surface sizing on watercolour paper helps watercolour and acrylic paint sit brightly on the surface rather than be absorbed into the fibres where it will look dull and washed out, and on canvas it prevents oil paint coming into contact with the fibres which would slowly be damaged by the oil. Size is also often a stiffener for canvas, so it will wobble less. Wood panels do not need to be sealed from oil for longevity but sizing gives a better surface for oil grounds or genuine gesso grounds to sit on by evening out the absorbency of the wood which would otherwise have different absorbencies between the grain lines and the wood between them. Sealing the wood also prevents moisture from drawing up colour, acids, glues, etc. from the wood into your gesso.
A size can be made from acrylic polymer, PVA, acrylic and casein, or animal gelatine. Be aware that there are over 50 different types of formulations of PVA (Polyvinyl Acetate) and only a few are of a good enough quality to make permanent paintings because they have low acidity, retain flexibility, and have low colour change. Look for pH neutral, sometimes called acid-free, PVA. PVA not made specifically for artists will deteriorate and is to be avoided. We stock canvas that is partially prepared, it is sized with rabbit skin glue ready for application of an oil ground or another coat of glue if you wish it to be clear coated. You can read more about sizes in Rabbit Skin Glue: Preparation, Uses and Alternatives.
This shows that oil has slowly seeped out of the paint and created a halo on this un-primed paper.
These are just a few of the wide range of available acrylic primer/grounds.
A primer is a glue that sticks well to your surface, better than your paint