Oil paint should never go onto bare canvas. The linseed oil that binds oil paint contains linolenic acid, and over years it breaks down the cellulose in cotton and linen fibers. Conservators call the result oil rot: the canvas turns brittle, then brown, then it cracks. A primed surface is the barrier that stops it, and that is the real reason to prime. Better adhesion and a smoother surface are genuine benefits, but they are secondary to keeping the painting from destroying its own support.
Size, primer, gesso, ground: four words for three things
These terms get used interchangeably, including by people selling the products, and the confusion causes real mistakes. Here is what each one actually means.
- Size seals the fibers of the fabric so nothing soaks through. It is a sealer, not a paint layer, and it is usually clear. Modern practice uses a PVA size.
- Ground is the layer you actually paint on. It is opaque, it gives the surface tooth for the brush to grip, and it controls how absorbent the surface is.
- Gesso in modern usage means acrylic gesso, which is an acrylic dispersion ground. It does the sealing and the ground job in one product, which is why most painters only buy one thing.
- Primer is the general word covering all of the above. It is also the word on the can at the hardware store, which is where most of the trouble starts.
True gesso, the traditional kind made from chalk and animal glue, is rigid and cracks on flexible canvas. It belongs on wooden panels. What sells today in a tub labeled gesso is not that material at all.
What to actually use
Acrylic gesso
This is the standard for most painters and it is what to buy if you are not sure. It goes on white, dries in under an hour between coats, cleans up with water, and it seals and grounds in a single product. Golden, Liquitex and Winsor and Newton all make one. It works under both oil and acrylic paint, which is convenient if you switch between them.
One honest caveat worth knowing rather than worrying about: on a large flexible stretched canvas, some conservators flag a long-term risk that an oil layer and an acrylic layer age at different rates and eventually separate. On rigid panels the question does not arise. For most work this is a theoretical concern, but if you are painting something intended to last a century, an oil ground is the more conservative choice.
Oil ground
An oil ground is the traditional option and gives a smoother, less absorbent surface that many oil painters prefer to work on. Gamblin Oil Painting Ground is a common example. The tradeoff is time: an oil ground needs a size underneath it, and it needs several days to cure before you can paint, not several hours.
Alkyd primer
Alkyd primers made for artists sit between the two. They give an oil-like surface but dry considerably faster than a traditional oil ground. They are worth knowing about, and they cost more.
What not to use, and why
Search for primer for oil paint and most of what comes back is house paint. KILZ, Zinsser and the rest are excellent products for the job they were designed for, which is walls, trim, cabinets and doors. They are not artist grounds, and the differences matter:
- They are not made to stay flexible. A wall does not move. A stretched canvas flexes with temperature and humidity, and a rigid house primer will crack along with everything painted on top of it.
- Archival performance is not a design goal. Artist grounds are formulated for pH stability and non-yellowing over decades. House primer is formulated to be painted over within a week.
- Shellac primer yellows badly and does not clean up with water. Shellac dissolves in alcohol, not water. Brushes need denatured alcohol, or a fifty-fifty ammonia and water mix. Putting a shellac brush under the tap seizes the resin into clumps and ruins the brush.
- Oil-based house primer is slow, not fast. Listings and blog posts routinely claim the opposite. Latex primer is the quick-drying one. An oil-based primer generally wants overnight, and that is before any question of curing.
Three claims about primer that are not true
These circulate widely, including in articles that previously sat on this page, so they are worth naming.
Primer does not stop oil paint from yellowing. Oil paint yellows because linseed oil yellows, particularly in the dark and particularly under whites. That happens in the paint film itself, above the ground. A good ground keeps the canvas from discoloring the paint from below, which is a different and smaller problem.
A dark primer does not make dark colors darker. Toned grounds are a real and useful technique, but the reason to use one is to kill the glare of white while you judge values, and to let flecks of the ground show through as a unifying midtone. It does not deepen the paint sitting on top of it.
Mineral spirits do not make oil paint dry slowly. Solvents evaporate and speed the early stages up. Oil paint is slow because it dries by oxidation rather than evaporation, which is a chemical reaction with the air and takes days to months depending on the pigment and the thickness.
How to prime a canvas
- Start clean and taut. Wipe dust off with a dry cloth. If the canvas is loose on its stretcher, adjust the keys before priming rather than after, because a coat of ground on a slack canvas will crack when you tighten it.
- Apply the first coat thin. Thin the gesso slightly with water if it drags. Work in one direction across the whole canvas and do not try to get full opacity yet. A thick first coat sits on the surface instead of soaking into the weave, and it will peel later.
- Let it dry, then sand lightly. Fine sandpaper, a light hand, and a wipe with a damp cloth afterward to lift the dust. Skip this if you want the canvas texture to show in the finished painting.
- Second coat at ninety degrees to the first. Crossing the direction fills the weave far more evenly than two coats going the same way. Two coats is enough for most work. A third gives a smoother surface if you are painting fine detail.
- Give it real drying time. Acrylic gesso feels dry in twenty minutes and is genuinely ready in a few hours. An oil ground needs several days. Painting into a ground that has not cured is how you get a surface that never quite sets.
What about rabbit skin glue
Older instructions call for sizing canvas with rabbit skin glue, and it is the traditional method. It is also the one modern conservators most consistently advise against. Rabbit skin glue is hygroscopic, meaning it takes up and releases moisture with the weather, so it swells and shrinks under the paint film for the life of the painting. That movement is a well-documented cause of cracking and cleavage in older works. A PVA size does the same job without the movement.
If you are working in a deliberately traditional way and want the surface it gives, that is a reasonable choice made knowingly. It is not the default it used to be.
Does any of this apply to a paint by numbers kit
No, and that is worth saying plainly because people ask. A paint by numbers canvas arrives already primed and printed. Priming it yourself would cover the numbers.
Ledgebay canvases are pre-primed cotton, printed with the design and shipped rolled in a tube rather than folded, so there are no creases to work around. The paints are acrylic and vacuum-sealed, and each kit includes brushes in a range of sizes and an 11 x 17 reference print. If you want the longer explanation of what that primer layer is doing under the print, our guide to what gesso is covers it.
The one place priming does come up with a kit is if you decide to paint something of your own afterward on a plain canvas, which a lot of people do once they have the brushes out. In that case everything above applies.
Related reading
- Layering acrylic paint, for how coats build on a primed surface
- How to frame a paint by number canvas
- Paint by numbers for beginners
- The full kit catalog, and custom paint by numbers from your own photo
