Sizes & Grounds

General Comments

Background Notes:

This page covers the topic of sizing and priming supports, and includes some recommendations on what materials to use and why, and how best to apply them.  Check out the resources at the bottom of the page.

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.

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.

Virgil’s Assessment:

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.

Those artists who purchase pre-primed linen should determine the nature of the ground used. If the painting you intend to paint is important to you, my suggestion is not to paint it on a canvas that has zinc oxide in its oil priming. [TBD: Link to our page warning about zinc in grounds and in paints]

Where permanence and longevity are concerned, traditional oil grounds are the very best for use with rigid supports or canvas glued to them.

Artists who purchase prepared supports may want to add another layer of ground on their own. If you do so, allow plenty of time for the new ground layer to cure, even with acrylic grounds, for optimal performance and longevity. Water escapes slowly from water-based grounds/primers, and there are other ingredients in acrylic grounds that migrate to the surface with the water but do not evaporate, thus they end up on the surface. It’s a good idea to clean the surface of them before painting on these grounds with oil paints.

Size and Sizing

Definitions and Types of Sizes

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.”

Advice for Sizing

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.

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 an acrylic sizing, follow Golden’s latest recommendation of which acrylic product to use, at what strength, and how many coats.

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.

Definitions and Types of Grounds

Ground: a priming layer applied to the support on top of the size. Its main purpose is to seal and prepare the surface to accept subsequent layers of oil paint, allowing for proper adhesion.

Other benefits of grounds:  they counteract the uneven absorbency of  supports; they help buffer the effects of humidity on supports; they can make the paint application easier and consistent; they can be toned for aesthetic effects; and when the grounds are white, they can reduce the visual distortions from paint that becomes transparent over time.

Priming: the act of applying a ground.

Gesso: a traditional ground made of melted animal collagen glue mixed with a white pigment such as calcium carbonate, calcium sulfate, or gypsum. It should be applied only to rigid supports.

Traditional Oil Ground:  a priming layer, white or light grey usually, with linseed oil or alkyd resin as the binder.  It is quick drying and lean.   It remains the very best as far as long-term performance and durability are concerned.

The best oil primer is white lead in linseed oil, with perhaps a minor addition of calcium carbonate. Together these ingredients make a leaner ground than any of the paints that will go over it.  “Leaner” means it has a lower percentage of oil in it than any of the oil paints you will be applying on top, in perfect accord with the fat-over-lean dictum.

Acrylic Ground:  A modern alternative to traditional oil grounds for priming. A good acrylic ground has
sufficient acrylic levels to yield excellent flexibility, while also having sufficient pigment levels to create a favorable level of absorbency and opacity. There should be enough water to ensure proper shrinkage to tighten a stretched canvas, yet be resistant to cracking when flexed. (Golden offers a line on acrylic grounds).

Virgil notes:  manufacturers often incorrectly acrylic grounds  as”gesso,” which creates confusion. Real gesso is not flexible enough to be a good ground on stretched canvas, but acrylic grounds are. Since they are different materials with different properties and applications, they should not be called by the same name.

Fredrix is one of the better brands of pre-stretched canvases. Their acrylic ground is a heat-set acrylic, which is probably as good as an acrylic ground can be.

Advice for Priming

Ground layers should be applied evenly and thinly. Sanding between coats can be done to produce a smooth finish.

According to MITRA, oil, acrylic dispersion, and alkyd grounds can be applied to either fabric or rigid supports. But traditional gesso, which is relatively brittle, should only be applied to rigid supports; flexible supports with gesso as the ground are more susceptible to cracking and other problems in response to changes in the environment.

The very best situation for oil paintings is a lead ground bound with linseed oil, because it imparts extra durability, film strength and flexibility to all the oil paints applied on top of it. If the white paint used is also lead white, so much the better, as far as long-term performance is concerned.

Gamblin Ground is NOT water-based and requires adequate wait time before paint layers are applied.

Priming Canvas Supports: 

Trying to prime stretched canvases with traditional gesso is a bad idea, because it is not flexible. Real gesso is a good ground for rigid wood panels, but not for stretched textiles.  Gesso is not a suitable ground for stretched canvas because it lacks sufficient flexibility

Historically, painters experimented with adding linseed oil, honey, and perhaps other ingredients to glue-chalk gesso to make it more flexible for use as a ground on canvas. Those experiments generally did not work out well, and the practices were discontinued after it was discovered that white lead in linseed oil was a better ground than glue-chalk gesso for paintings on stretched canvas.

Priming ACM Panels: 

ACM panels should be primed first with an acrylic or alkyd primer that has been tested for adhesion on that surface. Not every acrylic or alkyd primer may be suitable for use on ACM. Golden has tested their acrylic primers on ACM, and Natural Pigments has tested Rublev Colours Lead Alkyd Ground on ACM.

Priming Copper Panels:

Historically, oil paintings on copper have been done using an oil ground of white lead in linseed oil or painted on in oils directly on the copper. These paintings have generally held up quite well. I would be leery of applying any water-based ground such as acrylic to copper, because it has no long-term history to assure me of its soundness. Thus it’s essentially a gamble with the future condition of the painting at stake, and unnecessary.

Resources

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)

A source for Lead Alkyd Grounds.