Usually, artists think about the choice of priming as they make their choice of support. The priming can be defined as the liquid application that, when dry, to some extent isolates paint layers from the support. This is necessary in the cases of wood and canvas, which, in their untreated states, are so absorbent that the oil binder would be sucked excessively into them, leaving a dull, mat paint surface and sometimes endangering the stability of the support. Metal, slate and marble, being virtually unabsorbent, do not require priming (which, if water-based, would not adhere well anyway), but these might require abrasion to give them a compensatory degree of tooth. Sometimes absorbent surfaces such as hardboard and natural hardwoods lack tooth, and light abrasion with fine sandpaper or silicon carbide paper is needed to take off the slickness of their surfaces.
The priming does not have to be a paint itself. For centuries, a glue made from rabbit skin, called size, has been applied as a preliminary layer to reduce, but not entirely eliminate, the absorbency of wood and canvas. It is possible to start painting a picture straight on to this, though most artists would prefer to add a pigmented layer of some kind in order to increase the luminosity of their subsequent painting. The choice of primer layer crucially determines whether or not a preliminary layer of size is necessary. There are three kinds of primer in common use for oil painting:
Oil- Based.
This is essentially a layer or two of oil paint made more mat than is encountered in most tube colours by ensuring that the oil content remains proportionately low, and/or, by adding sufficient quantities of turpentine (N.B. artists’ quality NOT decorators’) as a diluent. To satisfy the first condition it is wise to use paint which has been prepared with priming purposes in mind; Michael Harding makes such a blend as his “Foundation/Priming White”. More on this below. The important point is that a preliminary layer or two of size is ALWAYS needed when applying an oil primer to an absorbent support such as wood or canvas.
Gesso.
This is a kind of size-bound white paint most often applied in three or more layers to rigid supports such as wood. There is evidence from analysis of 16th century painting that it was applied to canvas as well, but modern authors divide as to whether it is wise to apply a preparation which dries so rigidly itself, to a flexible support: at any rate, it would be prudent here to apply no more than one, or at most, two layers. But with all kinds of canvas and most kinds of wood, gesso requires that the same preliminary layer of sealant size be applied to the support in order to avert the excessive absorption of the size binding the gesso itself, which would leave an underbound surface to the latter, liable to dust off. The possible exceptions to this rule are those few kinds of wood which are only minimally absorbent, such as hardboard which has been “tempered” with oil in manufacture. This latter product, however, seems to be virtually obsolete, and may not offer a good surface anyway for a water-based glue such as size.
Acrylic.
The most recently devised of the common primers, different kinds have been sold for over 30 years and their durability on rigid supports at least seems very probable. Some of them might present more absorbent surfaces than oil grounds, but this can be adjusted by recoating with oil primer. It is important to note that acrylic primer should NOT rest on a size layer; such a coating would damage the primer’s adhesion to the support. Acrylic primers which give a more toothy surface are often sold under the label “acrylic gesso” but in fact they bear no chemical resemblance to the true gesso described above.
It seems too obvious a point to make, but remember that whilst an acrylic primer can be overpainted with oil paint, acrylic paints CANNOT be applied successfully to an oil primer, since, being water-based, they would not adhere safely.