Rabbit skin glue is the traditional size for fabric support on panels. It seals porous fabric and isolates it from ground or oil paints.
Stretching the canvas is easy, because the size shrinks as it dries. It is mixed with water to make a strong adhesive. However, atmospheric changes may result in movement, causing aged oil paintings to crack
In traditional oil painting as practiced by the Renaissance painter, skin glue was used to coat the canvas. This is necessary because the linseed oil that forms the base of most oil paint contains an acid, lineolic acid, that will over time destroy the canvas fibers
When used in painting as a sizing, it is spread thoroughly over a canvas that has been placed on the stretcher. When the glue dries, it tightens the canvas. After this has been allowed to dry, an oil-based primer is then applied. When the oil primer has dried (keep in mind that oil paint dries slowly) the oil painter has an excellent surface upon which to paint. A canvas sized with rabbit-skin glue can be made tighter than with other alternatives such as an acrylic-based gesso because of the shrinkage. This type of canvas is also valuable because it can be sanded to a flatter texture, which allows the painter to achieve a finer level of detail than can be achieved with a typical acrylic gesso ground.
It should be noted that this rabbit-skin glue ground is only appropriate for use under oil paint. Acrylic-based media will flake off a canvas prepared with rabbit-skin glue and are therefore not appropriate.