Oil priming is a much more traditional way of prepping your surface for oil painting. Some people swear by oil priming—others say they don’t notice much of a difference. Although it provides a nice, slick surface, traditional Rabbit Skin Glue (RSG) will cause paint to yellow over time and because it is hygroscopic (it continually absorbs moisture from the atmosphere), it cause the painting to swell and shrink which in turns causes cracking. You can see this in very old paintings when you go to the museum. One of the reasons to prime your canvas with an oil-based primer is that oil paints stick to it better. Oil on oil provides better adhesion than oil on acrylic.
First, a size* must be applied to the stretched canvas to protect the fabric from the paint (linoleic acid, the component of linseed oil which causes drying, will burn cotton or linen fibers). The most traditional size is Rabbit Skin Glue. Hide glue must be prepared according to package directions. Brush glue on, making sure to drive it into the weave of the cloth, and cover the sides as well to avoid damage from oil. The result should be a glue layer that is uniform and covers all fibers, but without thick lumps that obscure the texture of the cloth. RSG (Rabbit Skin Glue) should be applied as a liquid, warm or hot. The surface should be free of pinholes (the result of air bubbles). If these are detected, a second coat is necessary. The canvas should be allowed to dry completely before applying a priming layer.
Modern, synthetic alternatives to RSG are available in the form of PVA Size. Check with your local art store and be sure to follow the instructions on the packaging before use.
Priming white (a simple mixture of chalk or gypsum, lead white, and linseed oil) can be thinned to a thick cream consistency with pure gum spirits of turpentine, taking care not to over-thin, and applied with a brush or knife following the long dimension of the canvas. When the first coat is dry to the touch, a second coat may be applied. The surface should be completely covered with primer, but not so thickly as to obscure the texture of the canvas. The canvas is ready to accept paint in 10-14 days. If the painting cannot begin within three weeks, the canvas should cure for six additional months to ensure a stable paint film.
For a more modern oil primer, try Winsor & Newton’s Oil Painting Primer or Gamblin’s Oil Painting Ground, both of which are non-yellowing, made with an oil-modified alkyd resin, and lead replacement so they aren’t toxic. Basically, companies are trying to reinvent old master techniques using modern science, taking out the bad parts (yellowing, cracking, toxicity) and replacing them with safer (for you and your paint) alternatives.
So to recap:
Traditional Oil Ground: RSG + priming white
Modern Oil Ground: PVA size + oil painting primer
*Size, when used in this context, refers not to physical dimensions but to a medium added to the canvas or linen. A size seals the porous fabric and isolates it from the ground and/or oil paints. Linen and cotton will prematurely rot without a size layer. Only fabric supports need sizing. Panels only need to have a ground. Acrylic gesso does not require a size.