Sizing is the sealant applied to canvas or paper before gesso or ground to prevent oil penetration. Learn the history of rabbit skin glue sizing and why PVA size replaced it.
Open any textbook on oil painting materials and you will find the same warning: never apply oil paint directly to raw canvas. The linseed oil in oil paint is acidic. Over time, it penetrates the canvas fibers, causes them to yellow and become brittle, and eventually rots them. Paintings made on unsized canvas develop cracks, tears, and paint loss within decades. The layer that prevents this is called sizing, and it is the first thing applied to a canvas before any ground or paint touches the surface.
Sizing is a sealant applied to a painting surface, typically canvas or paper, to isolate the support from the ground and paint layers. Sizing fills the pores of the fibers, reduces the absorbency of the surface, and creates a barrier that prevents oil paint and its acidic binder from reaching the canvas or paper beneath. Without sizing, oil paint soaks through to the back of the canvas, a problem called strikethrough, and the linseed oil begins to degrade the fibers from within. Sizing also tightens and stiffens canvas, creating a taut surface that holds paint better.
This entry covers what sizing does, the history of sizing materials, why rabbit skin glue was replaced by modern alternatives, and how to size a canvas today.
Sizing serves three functions. First, it seals the surface. A size layer penetrates the fibers of canvas or paper and fills the spaces between them, preventing liquid paint or ground from soaking through. This protects the fibers from direct contact with acidic materials like linseed oil, which causes yellowing and embrittlement over time. Second, sizing reduces absorbency. Raw canvas is highly porous. Paint applied to raw canvas soaks in unevenly, pulling the binder away from the pigment and leaving a weak, poorly adhered paint film. A layer of size creates a less absorbent surface that allows the paint to form a proper film on top. Third, sizing tightens the canvas. Animal glue sizing, in particular, shrinks as it dries, pulling the canvas taut on the stretcher bars.
The MITRA guide to adhesives and sizes defines sizing as "a material that is applied to a support to effectively seal the surface and serve as a barrier layer between the support and the paint and ground layers." The guide also notes that sizing helps prevent Support Induced Discoloration (SID), a phenomenon where water-soluble impurities in the support migrate up through the ground and paint layers, causing staining that becomes visible over time.
The traditional sizing material is rabbit skin glue (RSG), an animal glue made from the collagen in animal tissues. Despite the name, RSG is not always derived from rabbits. The term is used generically for animal hide glues, and commercial RSG often comes from cattle, pigs, or sheep. True rabbit skin glue, sourced from rabbits, has the highest bloom strength (a measure of gel rigidity) of any animal glue, typically around 550 grams. This high bloom strength means it forms a thicker gel and produces more shrinkage during drying, which tightens canvas more aggressively than other glues.
RSG has been used to size canvases for oil painting since at least the 16th century, when canvas replaced wood panel as the dominant painting support in Europe. The glue is soaked in water, heated until liquid, and brushed onto the raw canvas while warm. As it cools and dries, the glue gels and shrinks, pulling the canvas tight. The dried glue layer seals the fibers and blocks oil penetration. RSG sizing was the standard preparation for oil painting canvases for over 400 years, and it remains in use today among traditional painters and conservators.
The Golden Artist Colors Just Paint article on rabbit skin glue documents the properties and problems of RSG in detail. Its main advantage is the tightening and stiffening effect on raw canvas, which no synthetic alternative has fully matched. Its main disadvantage is that it is hygroscopic, meaning it absorbs moisture from the air. In humid conditions, RSG swells and softens. In dry conditions, it shrinks and becomes brittle. This cycle of expansion and contraction, repeated over years and decades, causes cracking and delamination of the ground and paint layers above it. Many old master paintings that show severe cracking in the paint film are suffering from the effects of RSG sizing responding to humidity fluctuations.
In the mid-20th century, conservation scientists began recommending polyvinyl acetate (PVA) size as a replacement for rabbit skin glue. PVA size is a synthetic polymer dispersed in water. It does the same job as RSG: it seals the canvas fibers, blocks oil penetration, and reduces absorbency. But PVA size is dimensionally stable. It does not absorb atmospheric moisture, so it does not swell and shrink with humidity changes. This eliminates the cracking and delamination problems associated with RSG.
The Gamblin PVA Size product page explains the conservation reasoning: "Conservation scientists recommend painters use neutral pH PVA size canvas and paper instead of rabbit skin glue. PVA Size provides a stable size layer that seals the canvas but does not swell or shrink like rabbit skin glue does. This is because PVA size does not re-absorb atmospheric moisture." PVA size is also pH neutral, which means it does not contribute to the acid degradation of the canvas fibers. It does not yellow over time, and it retains its flexibility indefinitely.
PVA size does not tighten canvas the way RSG does. This is the one property where the traditional material outperforms the modern one. RSG can pull a loose canvas drum-tight through its shrinkage during drying. PVA size has minimal effect on canvas tension. Painters who switch from RSG to PVA need to ensure their canvas is properly stretched before sizing, because the size will not tighten it further.
Acrylic-based sizing is a third option. Acrylic sizing is made from thin acrylic polymer dispersion, sometimes sold as "clear gesso" or acrylic medium diluted with water. It provides good oil blocking and moderate stiffening. Its main advantage is compatibility with acrylic gesso: since both the size and the ground are acrylic-based, they bond chemically and have the same expansion and shrinkage rate. The Utrecht Art Supplies comparison of modern and traditional sizings notes that acrylic sizing is fully compatible with oil, alkyd, and acrylic primers on both panels and canvas.
For PVA size, the process is simple. Apply one coat of PVA size to the front of the canvas using a brush, scrubbing it into the weave. If the canvas has a very open weave with visible pinholes when held to the light, apply a second coat to the back as well. Allow the size to dry completely, which takes about 24 hours, before applying gesso or oil ground. PVA size goes on thinly and does not fill the canvas texture, so the natural tooth of the canvas remains available for the ground layer.
For rabbit skin glue, the process is more involved. Soak dry RSG granules in water for several hours or overnight, following the manufacturer's dilution ratio. Heat the soaked glue in a double boiler until liquid (do not boil it, as boiling degrades the collagen). Brush the warm liquid glue onto the raw canvas. The glue gels as it cools on the surface, then dries to a hard, tight film. Two to three thin coats are better than one thick coat, because thick glue layers are brittle and prone to cracking. Sand lightly between coats, except for the final coat. RSG should only be used under oil or alkyd-based primers, never under acrylic gesso, because the hygroscopic glue layer will cause the acrylic ground to crack.
For panels, sizing is also recommended. Wood panels, especially those made from porous woods or manufactured hardboard, contain water-soluble extractives that can migrate into the ground and cause discoloration. Sizing both sides of a panel, plus the edges, helps prevent warping and reduces the risk of support-induced discoloration. The MITRA guide recommends sizing the back and sides of wooden supports as well as the front, particularly if the panel will remain unframed.
A properly prepared oil painting on canvas has a specific layer structure: canvas, sizing, ground (gesso or oil primer), paint, and varnish. Each layer has a distinct function. The canvas is the support that carries the painting. The sizing isolates the canvas from the acidic oil. The ground provides a white or tinted surface with the right absorbency and tooth for the paint to adhere to. The paint is the image. The varnish protects the paint and saturates the colors. If any layer is omitted or applied incorrectly, the painting's longevity is compromised.
The most common mistake amateur painters make is skipping the sizing step and painting directly on acrylic gesso applied to raw canvas. Acrylic gesso does provide some sealing, because the acrylic polymer fills the canvas fibers. But gesso alone is not a substitute for proper sizing, particularly for oil painting. The gesso layer is thicker and less penetrating than a dedicated size layer, and it does not isolate the canvas fibers as effectively. For professional-quality work, size the canvas first, then apply gesso or oil ground on top.
Sizing is invisible in finished paintings. You will never see it in a museum. But you can see its effects. Paintings from the 17th century that were properly sized and grounded, like Rembrandt's works at the Rijksmuseum, retain their structural integrity after 350 years. Paintings that were poorly sized or unsized show cracking, flaking, and canvas deterioration. The conservation departments of major museums spend significant time addressing problems caused by inadequate or degraded sizing layers in historical paintings.
For more on the layers that sit on top of sizing, read our entries on gesso, imprimatura, and ground. For the surfaces that sizing protects, see our entries on canvas and canvas panels. And for a practical guide to the full preparation process, read our post on oil painting techniques, or explore our guide to reading a painting.
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