There are four basic elements to the structure of an oil painting: (1) the support (e.g., canvas, wood, copper or aluminum panel), (2) the size layer, (3) the ground layer and then (4) the oil paint layers.
This page covers the topic of sizing supports and includes some recommendations on what materials to use and why, and how best to apply them. Be sure to check out our companion page on Grounds and also check out the resources at the bottom of the page.
For convenience, many artists purchase brand-name or generic supports from art suppliers with the size and grounds already applied to the canvas or panel. Artists who are more quality conscious might buy rolls of canvas pre-primed (with the size and grounds applied), and then cut out pieces that they then staple or tack onto their own stretchers bars or glue onto their own panels. Still others want full control over what they paint on. They might purchase fine quality linen that is completely raw and then size and prime it themselves before stretching it or gluing it onto panels. Some artists even avoid linen altogether. They prepare their own rigid supports, apply the size and grounds themselves, and then paint directly on them. Even artists who buy prepared supports or buy pre-primed linen should have some awareness about what type of size and ground has been used — that is, if they care at all about the longevity of their work.
What I paint on now is lead-primed linen canvas glued and tacked to rigid panels with basswood bracing around the edges, or polyester canvas glued to honeycomb aluminum or ACM panels. (See our page on supports for more information)
If I were to want to paint on stretched canvas, I would size it with one of Golden’s current recommendation for sizing products or two coats of Gamblin’s PVA Size, and then I would prime it with white lead bound with linseed oil. It would require re-stretching at least once during this process, and again later.
According to MITRA (see resource link at the bottom of the page), glues tend to form fairly rigid and brittle films and should not be applied too thickly to supports. Two or three coats of diluted size are preferable to one thick layer of glue.
The surface of the support should be sanded after each coat of glue size with the exception of the final coat.
Definitions and Types of Sizing
Size: A material applied to a support (wood, fabric, etc.) to effectively seal the surface and serve as a barrier layer between the support and the ground and paint layers. A layer of size reduces the absorbency of the support. Without it, oil from the ground and paint may soak into the cellulose fibers of the support and initiate their rotting. Some sizes also help prevent Support Induced Discoloration (SID), a phenomenon whereby water-soluble impurities in the support migrate up through the ground and paint layers. Size may also protect fabric supports from the acidity of certain materials (e.g., drying oils, alkyds) and create a more taut, planar surface (since fabric tends to shrink when sized).
Sizing: the act of applying a size to a support. Sizing precedes the application of the ground.
Rabbit Skin Glue (RSG): a traditional size that is dissolved in hot water and applied as a warm, liquid solution. RSG is not always derived from rabbits but may come from other animals such as cows, goats and sheep. Animal glues respond rapidly to changes in relative humidity, shrinking as the humidity drops and swelling and softening as it rises. (This is a quality known as “hygroscopic”.) The risks to your painting include: delamination and/or flaking of the overlying ground and paint layers; and also to planar deformations in the support. If used at all, RSG is best applied thinly to rigid supports (or canvas glued to one) to mitigate the expansion/contraction of the size layer. After an application of RSB, traditional gesso grounds rather than acrylic grounds should be applied because they are more compatible.
Polyvinyl Acetate (PVA) Size: Diluted with distilled water, PVA size is a contemporary size for fabric support. Conservation scientists recommend painters use neutral pH PVA size on linen and canvas instead of rabbit skin glue. PVA provides a good size layer that seals the fabric but does not re-absorb atmospheric moisture, swell and shrink like rabbit skin glue does. There are hundreds of different formulae of PVA. Gamblin’s offers a neutral Ph PVA Size.
Acrylic Polymer Dispersions — which consist of finely distributed particles of resin suspended in water and sometimes thickened into a gel form. Golden has a line of acrylic polymers (GAC 200, GAC 400) that has been tested extensively and found to be more stable than rabbit skin glue. According to MITRA, “Some consider acrylic dispersion sizes preferable to PVA sizes. They tend to provide a greater degree of stiffness, are less porous, and are more efficient at preventing support induced discoloration.”
Sizing Canvas Supports
Avoid rabbit skin glue as a sizing for stretched canvas. It’s not ideal because it is hygroscopic, and subjects the painting to fairly extreme stresses as it expands and contracts in response to changes in humidity. The inherent flexibility of canvas allows stress on the size to be passed along up to the oil paint film. With rigid supports such as wood panels, the chance of swelling is much lower, and only then is animal glue size more appropriate. (see Golden reference at the bottom of the page).
If you size canvas with neutral pH PVA, give it two thin coats.
If you use acrylic sizing, follow Golden’s latest recommendation of which acrylic product to use, at what strength, and how many coats.
(Update 2022/01/11: For sizing raw linen that you intend to stretch: you could use 1 coat of GAC 400 and then cover it with 1 coat of GAC 100 or GAC 200. GAC 400 is a paper and fabric stiffener. It does not protect the canvas from oil penetration. If you apply a second coat, that would make the canvas too brittle and prone to cracking. Source.)
Virgil advises applying a white lead/linseed oil ground after the sizing is completely dry.
Sizing Wooden Supports
The sizing of wood supports prior to applying acrylic grounds is best, to prevent the migration of wood color into and through the ground, a phenomenon known as support-induced discoloration (SID) and to reduce the absorbency.
According to MITRA, acrylic-based and vinyl-based sizes should be applied evenly to the back, front, and sides of wooden supports to prevent uneven warping. If animal glues are used, they should be applied the same way, but they will need to be coated with primer and paint and not left exposed to the environment.
For sizing wood, using a water-based material carries the potential for swelling of the wood fibers in reaction to the water. With pressed wood this would likely not be as great a concern as it would be with plywood and perhaps solid wood panels, because pressed wood has no grain.
An additional consideration would be the possibility of problems arising if any water remains in the wood or sizing when the oil ground is applied over them. Water is slow in escaping from absorbent substances such as wood. Adequate drying time is needed.
Even if Virgil is gluing canvas to a wooden support, he seals the wood first with polyurethane. Water causes wood to swell and sometimes warp. PVA is water-based. The top veneer of plywood is thin and reacts badly to water. Some plywoods are better than others for this application, but I doubt any of them are ideal as supports for oil paintings unless canvas is glued to them, and the painting is done on the canvas.
Additional advice from Sarah at Golden: An alkyd on wood is a perfectly fine size. Non-water-based coatings have the advantage of not causing swelling of the wood or raising of grain if using a plywood.
More advice from Scott Gellatly of Gamblins: Galkyd thinned 1:1 with Gamsol, brushed on and then wiped off, does an excellent job of reducing the absorbency of wood panels and reducing the amount of moisture that the panel will pull in through its life. Another important note is that a sealer should penetrate the support and not sit on the surface as a discrete layer. The wiping off of the Galkyd/Gamsol is an important step, as it should not dry with any added gloss as you don’t want to undermine the adhesion of the ground.
Here are some valuable resources from reliable sources:
Adhesives and Sizes Advisory by MITRA (Material Information and Technical Resources for Artists) — PDF document
Grounds and Primers by MITRA — PDF document
Grounds for wood panels — This Natural Pigments article provides recipes for sizes and grounds, and step-by-step instructions on how to prepare and apply gesso to wood panels.
Emulsion grounds — another Natural Pigments article on creating and applying emulsion grounds, which typically consist of an emulsifying adhesive, such as animal collagen glue and vegetable oil with chalk and lead white. They can be applied to all kinds of supports.
Preparing a Canvas for Oil Painting (Advice on Sizes and Grounds from Gamblin)
Preparing a Painting Support (Advice for Sizing and Priming from Golden)