Canvas - Which Surface?

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 they like the raw coloration of the linen 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 (or sized) 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 known 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.

An oil-primed canvas is the traditional surface for artists working in oils. The 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 consists of white (flake white) pigment ground into linseed oil or an alkyd resin binder. Because of the high content of linseed oil, traditional oil primer can be susceptible to yellowing when kept in the dark; but it brightens up again when exposed to sunlight.

Is it OK to use oil paint on acrylic-primed canvas and vice-a-versa?

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.

An oil-primed canvas can only accept oil paints. Although oil paint can be applied to an acrylic gesso primer, acrylic paint will not permanently adhere to an oil-primed canvas and will eventually peel off. 

What is unprimed linen or cotton?

Unprimed linen or cotton canvas does not include any treatments, glues, sizings, or priming layers whatsoever. You have the freedom to treat, stretch, and prime the canvas to your specifications.