Photo thanks to Anita Thomhave Simonsen

Size, primer and gesso. If you're anything like me, you're probably as confused as *** when it comes to these and preparing a surface for painting. Not any more. This post aims to clean away the mud and is a companion post to the one last month on painting substrates. So if you want to know what substrate is usable and whether it requires sizing or priming or both, check it out. This one is about the actual size and primer options. And some of it is straight from the wonderful AMIEN forums. Go check them out for all your technical artist materials questions.

Why Size?

Okay, here we go. Raw canvas, or any oil painting substrate, requires a barrier between itself and the oil paint. This is to prevent the paint sinking in to the surface (makes for terrible colour) and also to prevent the oils in the paint attacking and rotting the substrate (a nasty habit of oil paints).

In the case of acrylic paints, a barrier between the paint and substrate is required to prevent Support Induced Discoloration (SID) which is something that occurs in acrylic paints and mediums only. Many common artist supports have impurities that can discolor a translucent acrylic gel layer or color glaze, and a barrier must be applied to ensure the products stay clear as the films dry.

As an acrylic paint film cures, the water exits two ways: through the surface of the paint and through the back of the support, if it is porous enough. Canvas, linen, wood and masonite are all porous enough to allow water to absorb into them. During this drying process, the water is actually in equilibrium moving back and forth between the acrylic paint and the support. The water extracts water-soluble impurities such as dirt, sap, starches, etc., from the support and deposits them into the acrylic film. The result is a discolored (typically amber) film, with the degree of discoloration dependent on the amount of contaminants deposited and the inherent level of inpurities in the support.

SID contamination often goes undetected. In most cases, the paints applied contain a sufficient level of pigment, thus a strong enough color, to conceal the yellowing. However, in a transparent glaze and especially in thick translucent gel layers, SID becomes quite noticeable. SID can transform the appearance of an Ultramarine Blue glaze into a lower chroma, greenish color. Gesso alone will not stop SID, and different gels and mediums have varying degrees of blocking capabilities.

The acrylic dispersion mediums, paints and primers do absorb and expell moisture for about a year after they are applied -- or less time if the environment is warm and dry. After that's finished the resins coalesce into a more continuous film. They are still susceptible to moisture penetration, but not to the degree that puts them at the same risk as RSG. And it's preventable by coating the finished paintings.

In summary, for both oil and acrylic paints, it is wise to put a barrier between the paint and it's substrate. This barrier is known as size. It is meant to be a penetrating sealer, not a coating. It is therefore very thin, using it is good practice, and it's very easy to do.

Ancient and Modern Size

Traditional size for oil paints is a (disgusting I say) concoction of rabbit skin collagin heated with water. Although this has been used for hundreds of years it is know known by conservators to cause more problems than it solves. How? RSG is hygroscopic. It continuously absorbs moisture from the atmosphere, causing it to continuously swell and shrink. Over time, this constant flexing causes the oil paint on top, which is quite brittle, to crack. In fact, RSG is now believed to be the main cause of cracking in old oil paintings.

Instead of RSG underneath oil paintings, it is now recommended to use a pH neutral PVA or any acrylic medium whose manufacturer recommends it as a size. Please remember that PVA size or acrylic medium does not tighten fabric like rabbit skin glue.

Underneath acrylic paints, the best choice is acrylic medium whose manufacturer recommends it as a size that can block SID - a blocking size. A blocking size will be formulated to inhibit the migration of soluble organic materials from the support through to the ground. Two that are often mentioned are Gamblin’s PVA size and Golden Acrylics’ GAC100. Liquitex makes one as well, as I am sure others do too.

Two coats of size on the front of a substrate are sufficient. They should be touch dry and not cold to touch (which indicates moisture) before the next layer is added.

Why Prime?

Priming is the adding of an absorbent coating to a substrate. The aim is to provide the paint with a porous surface to adhere to. Primer is also known as ground or gesso. It is not size and will not seal or create a barrier to the paint. It does exactly the opposite. It is absorbent and provides tooth, or texture for the paint. It also tends to stiffen the substrate.

Usually two coats of primer is used.

Just for the record, you don't need to prime unless you want the surface that priming gives. You can paint directly onto modern size (but not RSG).

All About Gesso

Traditional gesso

Real or traditional gesso is for hard inflexible surfaces only. It is too brittle for canvas and will crack. Real gesso is like a plaster and is ideal on wood panels. It is made from a thin base of RSG and inert white pigments. The pigment is powdered chalk, calcium or gypsum. Sometimes titanium white pigment is also added to the mix for brightness.

This zinc oxide in the ground is a problem in terms of archival quality. Paint over zinc oxide and within a few years time the painting is  likely to be delaminating or cracking.

Modern gesso

Gesso changed in the 20th century. In 1955, Liquitex, an acrylic paint company developed the first water-based acrylic gesso. It provided a consistent and inexpensive primer layer for both acrylic and oil paintings. Technically it is an acrylic dispersion primer, not a gesso, but it is mostly known as acrylic gesso or acrlyic primer. This is the only choice of primer available for use under acrylic paints.

Oil gesso

It's really useful to remember that an oil-primed canvas can only accept oil paints. Although oil paint can be applied to acrylic gesso, acrylic paint will not permanently adhere to an oil-primed canvas. The acrylic gesso will eventually peel off the oil-primed canvas.

Today there are two alternatives to traditional gesso with it's RSG content. There are oil/alkyd grounds which are the quickest drying option. And there is lead white oil primer. The latter is somewhat hazerdous to use. Don't smoke or eat in your studio. Use a barrier cream on your hands or wear thin rubber gloves so the lead can't penetrate your skin. Use odorless mineral spirits to thin the primer, not gum turpentine; odorless mineral spirits is hazardous but not as hazardous as regular mineral spirits and definitely not as hazardous as gum turpentine. In any event, you will want good ventilation when using these solvents: at least work near an exhaust fan. Thoroughly wash your hands when you are finished.

For the lead white oil primer you will have to apply at least 2 thinned coats. Allow perhaps a week for the drying of the initial coating, depending on the temperature and humidity of your studio. If it is dry to the touch after a few days, then you can go ahead and apply the second coat. After you apply the second coat, check to see that the coverage is good. You might want a third coat.

So there are three choices for priming for oil paints. Acrylic gesso and two kinds of oil based gesso.

Acrylic grounds under oils

In recent years, some artists have begun to question whether or not acrylic gesso is the right product to use under oil paint. Acrylic dispersion grounds retain their flexibility as they age, and the oil/alkyd grounds get stiffer and more brittle as they age. There is concern that this difference in flexibility may cause oil paintings on acrylic grounds to delaminte as they age.

The best current knowledge is that they are considered very good grounds for oil paints, with two caveats. 1.) You must purchase the highest quality ground you can find. That means: do not by acrylic dispersion grounds by price, and be sure the label tells you what's in the stuff. If you're unsure about the latter, call the manufacturer. 2.) Oil paints ought to be painted on rigid substrates. If the acrylic dispersion ground is applied to a panel, or to a fabric mounted on a panel, there should be no problems.

For a better bond with oil paint, after the acrylic primer is dry (when it's no longer cool to the touch is a good indicator) you should wipe this surface down with warm water and a clean rag to remove surfactants and allow to dry again (24-48 hrs) before applying oil based paints.

Linky Goodness

Amien forums

Golden Paints technical info

Gamblin & size

Liquitex pdf book on acrylics

More art technique articles

This article is one in an ongoing series of technical articles for artists, all archived together and accessible from here. The topics range from details on materials, to the business of art, to specific art techniques. Please make use of this resource.

Thanks to Cara B Anderson for the photo.

Not all canvas 'for artists' is actually any good. Here's my notes from educating myself, written up into a readable form.

Rolled and Stretched and Primed

Canvas for artists is generally available in three forms. Stretched (and primed), unstretched and primed and unstretched and unprimed.

Naturally the form that saves an artist the most time is also the most expensive as all the preparation work has been done. I am talking here about the prestretched and preprimed canvases. There is nothing to do but take any wrapping off and get to being creative. This is a time saver that is not to be underestimated. The down side of ready to go canvas is that you have to take what's on offer. Which is not necessarily so bad. However, if you are after a non standard size or are particular about the weight of canvas on which you work or fussy about the gesso used and the surface finish you like to paint on, then the small number of options of prestretched canvas will probably drive you nuts.

So on to unstretched canvas. This is usually priced per length from a roll and cut when you buy. There is a lot more choice with this form of canvas, but some preparation work before painting.

Unstretched canvas does not have to be stretched onto a frame, but that is the conventional way. It is perfectly possible to work on canvas as you do a piece of paper. The stretching is to give a nice flat and smooth surface to paint on. Most painters like the bounce or give in a canvas stretched on a frame. I'm not going to give a how to on stretching canvas right here, but just say that if you buy canvas off a roll, you are more than likely going to stretch it onto a frame yourself. That takes time, of course. But you can make the frame any crazy size you want.

Canvas bought from a roll comes in two forms. Primed and unprimed. Primed means that once you've managed the stretching, you can get right to painting. Probably a good choice for an unusual sized frame where you're not too fussy about the gesso primer and the finish of the surface you paint on. Although it is certainly possible to reprime and/or sand a preprimed canvas to your heart's content.

The most labour intensive, for the artist, is to use unprimed unstretched canvas. Here you will be both stretching and priming your painting surface. If you are super particular about your painting surface, this one's for you. Though be warned, you will spend about half your 'painting time' in stretching and priming. But you will be able to get *exactly* what you want.

But There's Canvas and There's Canvas

That was meant to be a quick rundown on the forms that art canvas comes in. There is also variation in the actual canvas itself.

Jute or hessian

Jute or hessian is dirt cheap, has a very rough and open weaved texture and quickly weakens and becomes brittle with age. Not recommended for longevity! But what a texture.

Synthetic canvas

There are synthetic canvases usually made from polyester. Photo or giclee printing onto canvas tends to use synthetic canvases. They tend to have a much more even, tight weave which lacks character. Or bestows supreme consistency, depending on your taste. They are said (by the manufacturers) to stand up much better than cotton and linen when exposed to the elements and to be immune to bacteria, mildew and air pollution. Professional art supply stores call synthetic canvas 'cheap' (which is quite an advantage, I think) and tend to take a sneering tone toward it. I have not managed to turn up any unbiased information on it's artistic archival properties (ie, how it reacts over time to gesso and paint), however the tent and boating industries sing the praises of synthetic canvas, citing it's longevity over traditional cotton canvas. And if it performs in those tough environments, the omens seem good to me. One to watch.

On blended yarns: "Blends of fibre (ie cotton/polyster or cotton/linen) should be avoided as the final product is usually unstable due to the different weights, strengths and characteristics of the two yarns. They are only suitable if the blend is in the thread, and exists in equal weight ratio in the warp and weft." Thanks.

Cotton duck

This is the one to go for if you are buying cotton canvas. It is more tightly woven than plain canvas. Cotton canvas is the most conventional surface for painting (oils and acrylics obviously). It's properties are well known and it's available everywhere. It is flexible, easy to stretch properly and not too expensive. For large paintings it is too flexible. Large being over a metre square.

Linen

From the flax plant. Belgian is considered the best. If you are after absolute quality, this is the type of canvas to go for. Although linen is difficult to prime and stretch properly, it has the best archival properties. Because it has less flexibility than cotton, it expands and contracts less with changes in temperature and humidity. This means the paint on top has a much more stable surface and is far less likely to crack. And it's strong and stiff enough to support large paintings (over 1m square). Cotton fibres are short and flat (only 4 to 5cm long) whereas linen fibres are round and can range from 25 to 90cm in length. Many painters also like the uneven, natural texture of linen. Oh, and by the way, it's brown.

How Much Does It Weigh and Why You Might Care

Just to be clear here, I'm talking about cotton and linen. The synthetic canvases are much stronger per weight, so these figures don't apply.

7-8 oz. Beware! Not good enough for long lasting art. Often used for prestretched canvases. Usually poor quality loose weave and not very stable or strong. Avoid.

10 oz. The world's most popular art canvas weight. Fine for small artworks or situations where there will be little strain on the canvas. The main thing to watch out for is that most cotton canvas of this weight is twice as strong in one direction than the other (2:1 weave). Which reduces its dimensional stability. Warp and weft threads should be of equal weight, strength and material. The ideal yarn is closely and tightly woven with a square (1:1 or 2:2) weave.

12-15 oz. The best weight for painting. Nice and strong. Now you know.

More art technique articles

This article is one in an ongoing series of technical articles for artists, all archived together and accessible from here. The topics range from details on materials, to the business of art, to specific art techniques. Please make use of this resource.