What is the difference between sizing and priming?
Traditionally, all canvases are first sized and then primed. Size is a clear barrier of rabbit-skin glue or a synthetic equivalent such as acrylic polymer gel that seals the surface of the cotton or linen. This prevents additional layers of primer and paint from coming in contact with the fibre, avoiding weakening or rotting. Some artists paint directly onto clear-sized linen because the like the raw coloration of the linen as as a working surface, although it's generally advisable to add an extra layer of sizing to commercially prepared surfaces.
A primer, gesso or ground is a (usually white) coating which provides a surface the paint can adhere to. Usually two to three layers of primer are applied by hand or by machine to pre-primed canvas. You can prime raw cotton or linen from scratch or add additional layers of primer and then sand the layers back evenly if you require a smoother surface.
What is the difference between acrylic- and oil-primed canvas?
An acrylic-primed canvas is primed with acrylic gesso, also know as acrylic ground or universal primer because it is also suitable for oil paint. Acrylic gesso differs in absorbency, consistency and quality from brand to brand. Typically, it consists of a calcium substrate (gypsum in the Italian tradition, chalk in Northern Europe), combined with an acrylic polymer medium as binder, pigment (such as titanium dioxide) as a whitening agent, and other chemicals that ensure flexibility.
Acrylic-primed canvases are absorbent, creating a surface to which water-based acrylic paints can bind. It is best to size the canvas with rabbit-skin glue or an acrylic medium such as Golden's GAC-400 before priming. It is common to have two to four layers of a two to four layers of acrylic gesso, and advisable to work in thin layers. Each layer should be allowed to dry completely before the next is applied.
Gesso is useful for the preparation of any surface, not just canvas. It will also adhere to masonite, MDF or unpainted wood, but not to oil-based surfaces.
Inexpensive student-quality gesso tends to be very porous, causing pigments to leach into the surface and lose its vibrancy when it dries. If you find this is a problem, it's worth seeking out an artist's quality gesso or trying some good quality pre-primed canvas.
An oil-primed canvas is the traditional surface for artists working in oils, although many oil painters now use universal-primed canvas instead because it is easier to find, less toxic and less expensive. This surface is naturally very flexible, tough, completely non-porous, and "lean", i.e. appropriate as the first layer in the fat-over-lean oil-painting technique.
Oil-primer traditionally consists of white lead (flake white) pigment ground into linseed oil. But because white lead is highly toxic and associated with lead poisoning, its production has been banned in most countries. Flake white and lead-based primers are now hard to come by, and the health risks involved in using these products should not be overlooked.
The good news is that there are alternatives. Van Mulders is a Belgian group specializing in oil-primed canvases using lithopone white (a sulphide of zinc white) and other products as a substitute of white lead. And the American company Gamblin produces a ground for oil painters with a binder of alkyd resin rather than linseed oil and titanium dioxide for pigment. Gamblin's ground dries within a week (traditional oil-primer takes several months) and is less likely to yellow. Because of the high content of linseed oil, traditional oil primer can be susceptible to yellowing when kept in the dark; but don't worry, it brightens up again when exposed to sunlight.
Is it OK to use oil paint on acrylic- primed canvas?
It is generally accepted that it is fine to paint using oils over a good quality acrylic or universal primer. However, there are a few issues to be aware of. Solvents commonly used in oil painting, such as turpentine or odorless spirits, can leach through an inexpensive, thin acrylic primer and damage the fibres of the canvas. Additionally, if oil paint soaks into a surface that has not been properly primed or sealed, it can infiltrate the canvas and remain damp, deteriorating at the fibre.
For these reasons, it is worthwhile to ensure that the surface you use is of a suitable quality that has been sized and then primed with at least two layers of artist's quality gesso.
