(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987)
Priming is done to regulate surface absorbency. Traditional gesso panels, for example, are often too absorbent and metal supports are too nonabsorbent. The challenge is to regulate the absorbency to your needs. There are various ways to accomplish this, but one rule must be observed: never apply a waterborne paint over an oil-primed surface. Do not be fooled by appearances. It is not difficult to get waterborne paints to go on an oil-primed surface, but you cannot make them stay there over the years.
Waterborne priming to reduce absorbency can be accomplished easily using diluted polymer medium (half water to half medium) brushed on in thin coats or, better still, airbrushed. The Winsor & Newton Prepared Size can be heated, diluted with water, and applied in thin coats until the desired absorbency is obtained. Dilute solutions of gum arabic or a hide glue can accomplish the same effect.
Oil-based priming to reduce absorbency is most often done with a retouch varnish, which can be brushed on, airbrushed on, or applied with a commercial spray can. Dilute solutions of damar varnish or shellac are also used.
Metal support surfaces are made more porous by the application of several thin coats of a lead white paint (foundation white). Lead paints should not be airbrushed or sanded. Almost any foundation white paint, commercial metal primer, or lacquer primer (not synthetic lacquers) will do the job.