SIZE, PRIMER, GESSO AND GROUND EXPLAINED


Sometimes art terms can be confusing – some words mean more than one thing, or one product can be called two or three different names. A case in point is the words that we use for the materials and methods used to prepare surfaces for painting.

Although it is possible to paint in oil or acrylic on an unprimed support, you will find that there are some difficulties with paint handling if you paint directly on raw canvas, paper or wood. With acrylic paint, canvas can repel water so the paint beads up and doesn’t flow. With oil paint, canvas can soak up oil and produce halos of it around painted areas, or leave the oil paint crumbly and under-bound. With either paint, the weave could be more visible or bumpy than you want, or the colour of linen may be too dark; paper can absorb oil or buckle from water because it is too lightweight; wood can absorb paint unevenly in the stripes of the grain or absorption of water can cause it to swell or split. So there are treatments for these surfaces that are commonly used to make them easier to paint on. By using size to seal the surface followed by a primer, gesso or ground, an artist can create a surface that allows easier, more controlled painting. A second consideration is that these treatments also help create a more stable surface so the painting remains unchanged for longer. For permanent paintings you should be as concerned with the proper preparation of the foundation layers of the painting that are perhaps not visible (the support, the size and the ground) as with the layers that you do see (the paint, mediums and varnish).



Size, Primer, Gesso and Ground

Size, primer, gesso and ground are terms for the parts of the surface of a painting between the support – canvas, wood or paper – and the paint. Sizing is pretty clear – it is the first step of sealing the support. But there is some confusion about the other three terms – primer, gesso and ground – which are often used interchangeably, though not always correctly. Materials manufacturers are not consistent with naming, which can add to the confusion. Though some of these terms also apply to surfaces for works in dry media like pencil and pastel, and to other painting media like watercolour, egg tempera and encaustic, I am mostly focussing on how they are used in oil and acrylic painting because there is a wider range of choices available, so there is a bit more confusion. And the other media can be extrapolated from this information.


Sizing agents include: acrylic size, acrylic polymer used with or without acrylic fabric stiffening medium, acrylic matt medium, PVA size, and rabbit skin glue.



Size (sealant)

A size is a glue that seals the surface to reduce absorption. The surface sizing on watercolour paper helps watercolour and acrylic paint sit brightly on the surface rather than be absorbed into the fibres where it will look dull and washed out, and on canvas it prevents oil paint coming into contact with the fibres which would slowly be damaged by the oil. Size is also often a stiffener for canvas, so it will wobble less. Wood panels do not need to be sealed from oil for longevity but sizing gives a better surface for oil grounds or genuine gesso grounds to sit on by evening out the absorbency of the wood which would otherwise have different absorbencies between the grain lines and the wood between them. Sealing the wood also prevents moisture from drawing up colour, acids, glues, etc. from the wood into your gesso.

A size can be made from acrylic polymer, PVA, acrylic and casein, or animal gelatine. Be aware that there are over 50 different types of formulations of PVA (Polyvinyl Acetate) and only a few are of a good enough quality to make permanent paintings because they have low acidity, retain flexibility, and have low colour change. Look for pH neutral, sometimes called acid-free, PVA. PVA not made specifically for artists will deteriorate and is to be avoided. We stock canvas that is partially prepared, it is sized with rabbit skin glue ready for application of an oil ground or another coat of glue if you wish it to be clear coated. You can read more about sizes in Rabbit Skin Glue: Preparation, Uses and Alternatives.


This shows that oil has slowly seeped out of the paint and created a halo on this un-primed paper.



These are just a few of the wide range of available acrylic primer/grounds.



Primer

A primer is a glue that sticks well to your surface, better than your paint would. Then when your paint is applied, it sticks well to the primer, so then your paint is well adhered to the surface through a middleman. If your paint isn’t adhered well it could come off a bit when cleaned, rubbed or scratched, it could flake off after it has dried, or delaminate and come of in pieces when it undergoes a sudden temperature shift (the change from storage in a cool room to sitting in the hot sun waiting to be loaded into a van, for instance), is removed from the stretcher and rolled for storage, or if it gets a sudden knock from being dropped.

An acrylic dispersion ground will usually be called either acrylic gesso or acrylic primer and can fulfil the role of a size, primer and ground all in one. It seals the surface, it sticks to the surface even better than acrylic paint would and it provides a ground to paint on. It can be applied directly to a support without the need for a prior application of size. Most ready-prepared stretched canvases are Universal Primed, meaning that they have an acrylic ground so you can paint on them with oil or acrylic paint. There are also primers for particular surfaces, such as the Turner Glass Primer or Wood Primer for acrylic paint. The primer sticks to the glass and then acrylic paint sticks to the primer, where acrylic paint would not have stuck to the glass directly, it would peel off.

The verb most used for the activity of preparing surfaces is ‘to prime’. “I spent the weekend priming panels so now I have enough surfaces for the next six months of painting!” But ‘to gesso’ is also used. “I have to finish gessoing my canvas today because I need to start painting on it next week.”




Genuine Gesso is made of warmed animal glue and whiting. Shown here are four test pieces of wood with increasingly more coats of gesso right to left.



Gesso

The term ‘gesso’ is often used for all types of primers and grounds, because it was the original. But technically it only refers to the traditional mixture of hot animal glue and gypsum or chalk, applied in many thin layers to wooden panels while warm. It is pronounced with a soft g like gypsy or George, from the Italian for gypsum, a major component. Acrylic primers were created to be similar to traditional gesso. Because of the confusion of so many people calling acrylic primers ‘gesso’ most people differentiate the ‘warm glue’ type of gesso by calling it ‘true gesso’, ‘genuine gesso’ or ‘traditional gesso’. Genuine gesso is a hard, chalky surface built up of often more than 15 thin layers, that is sensitive to water and will crack if used on a flexible surface, so to prevent cracking it must only be used on rigid substrates. Sometimes a piece of muslin is layered onto one of the early layers to give some added strength to the gesso structure. Genuine gesso is extra-absorbent so it’s the best choice for painting in egg tempera or encaustic. Unless it states that it is traditional gesso, most things labelled gesso, like our Italian Plywood Gesso Panels, are made with acrylic ground and are not absorbent enough for egg tempera, encaustic or other techniques or mediums that require the very absorbent surface of genuine gesso in order to adhere. So be aware that there is some confusion in using the word gesso for these two different things. You can read more about genuine gesso and rabbit skin glue in Rabbit Skin Glue: Preparation, Uses and Alternatives.





Casein Gesso

Sinopia Casein Gesso is very absorbent and is the next best thing to genuine gesso. You use it straight from the jar at room temperature. It is made with milk protein (casein) which is a binder in casein paints. It contains a small amount of linseed oil that has been emulsified so it can be thinned and cleaned up with water. But the oil content is actually quite minimal, just enough to make the gesso flexible and water insoluble after it has dried and cured – because it is minimal they say that the surface doesn’t need to be sized first to protect it from the oil.

Once the gesso is applied it dries to the touch in a short amount of time so you can apply the second coat the same day when the first is touch dry, but no more than two coats a day. While the surface is stable enough for one additional layer, the oil content has to cure enough for subsequent layers to be applied. Because the recipe includes a linseed oil emulsion it will take four or five days for the surface to cure and become water insoluble. If you are working with aqueous wash techniques, which soak the surface with water, then you should wait for a complete cure. With less water or with oil techniques, the surface can be painted on after a few days.

Unlike genuine gesso you are not limited to rigid surfaces, though it should only be applied to smaller canvases and pieces of paper and they should never be rolled after the surface has been painted. We have had encaustic and egg tempera artists give it good reviews for absorbency.




Panels with different ready-prepared primers/grounds – top to bottom: Belle Arti gesso panel (acrylic primer), clear sized linen (RSG), universal primed linen (acrylic primer), oil primed linen and a universal primed cradled panel (acrylic primer).



Grounds

A ground is the surface that your paint comes in contact with. It’s the part that you experience with your brush. You may want it to be hard or soft, smooth or textured, absorbent or non-absorbent, coloured or white. The characteristics of your ground affect the way the paint handles, how easily it moves on the surface and how quickly it dries. Many acrylic texture pastes such as fibre paste, modelling paste or iron oxide, can be used as painting or drawing grounds. Grounds are of particular interest to oil painters for whom there is a wider selection of ready-prepared acrylic grounds, oil grounds, alkyd grounds, casein grounds (a mixture of oil, alkyd and casein, which is a milk protein paint binder) and others. There are also grounds for oil painting which are generally made by the artist just before applying, like genuine gesso, chalk ground, half-chalk ground, egg-linseed oil emulsion, and others, the recipes for which can be found in books on traditional oil painting or online. Grounds designed for pastel painting can be applied to a sized canvas for use with oil paint and some painters like the gritty texture.




4 Golden texture pastes used as grounds- clockwise from top left: Coarse Molding Paste, Molding Paste, Fibre Paste and Light Moulding Paste.




Oil grounds.



Oil Ground

Oil primer or oil ground is oil paint with added chalk to give some absorbency and some tooth for the oil paint to grab onto. It must be applied over sizing because applying oil primer is the same as applying oil paint, so the fibres need to be sealed from the oil. A stretched canvas, a canvas panel, or primed canvas by the metre, that has an oil ground will often be labelled as ‘oil primed canvas’. Pots of oil ground may be called oil primer, oil ground, alkyd ground for oil, alkyd primer, or thixotropic oil primer. Thixotropic is the quality of some materials to be thick while they are settled but flow once they are stirred, like jam, motor oil and alkyd mediums. Alkyd resins are created by modifying polyester resins with fatty acids. It’s the same fatty acids from the drying oils that are detrimental to cellulose which means you still need to size the canvas or paper to create a barrier so it can’t penetrate to the fibres. Alkyd usually dries faster than oil paint so a fast-drying oil ground is usually made with alkyd. A canvas with an oil ground usually appears yellow because of the dark yellowing of the linseed oil content. This doesn’t affect the painting if the entire canvas is covered with paint so that the canvas isn’t bare anywhere. Best practice is to use oil or alkyd grounds on a rigid support to prevent cracking, conservators generally recommend all oil painting be done on rigid supports.

Since we are located in the UK where lead is prohibited, we do not stock any lead oil primers. But I thought I would mention that they exist. A lead oil ground uses lead white pigment instead of titanium white, which affects the drying time and opacity.

For some time it was thought that oil painting on top of acrylic paint or an acrylic ground was not best practice because the oil paint would not adhere well. This concern has been pretty much discounted these days so long as the acrylic is absorbent enough for the oil to penetrate into it sufficiently to physically merge with it, which is the case of all artist-quality acrylic grounds, and acrylic paint is quite soft and porous.




Just a few of the absorbent/watercolour grounds available. Some come in colours or different “paper” textures.



Absorbent Ground

Acrylic paint can be used thickly to look similar to oil paint or in a more fluid manner similar to watercolour. If you use acrylics in a staining or washy way, you may find an Absorbent Ground or watercolour ground interesting to work on. It’s a bit like painting liquid paper onto your canvas or panel and can go on top of a ready-made, universally primed canvas to give it a paper-like surface. If using for oil paint be sure the surface is well-sealed before applying absorbent/watercolour ground. Some brands require an acrylic primer/gesso underneath and some don’t, so check the instructions. We have an article that covers these: Painting with Watercolour on Canvas: Choosing the best Watercolour Ground




Clear/transparent grounds.



Clear Grounds

If you wish to see the surface of the canvas, paper or wood instead of covering it with a white layer you can use a clear ground. This could be a few coats of matt medium, rabbit skin glue or a “transparent gesso” or “clear primer”. If you are preparing canvas or paper for oil paint you will need enough coats to be both the size and the ground. Be aware that a few coats of “clear gesso” usually creates a white haze from a build up of the material used to give it some tooth, so these are not usually actually clear. One way to reduce this effect is to seal the canvas with something truly clear like polymer medium before using anything that contains toothy material or matting agents and only use one of those for the final layer. This means there is less of the cloudy stuff built up and also since the canvas is now sealed, the clear part of the clear gesso doesn’t soak in leaving the matting material more concentrated on the surface.




Some acrylic dispersion grounds (acrylic primers/gessos).



Acrylic Dispersion Ground is an All-In-One Product

Acrylic ground (or acrylic gesso or acrylic primer) can fulfil all of these roles at the same time – it seals, primes and provides a ground. You can simply apply two or three thin coats, wait two days for it to cure if you are using acrylics and three days for oil painting, and you are ready to paint. But some artists prefer to add a size anyway before they apply acrylic primer, to reduce Support Induced Discolouration (SID) where sometimes the white ground gets yellow stains in it by drawing up colour from the canvas, wood or stretcher bars while it is wet. This is mostly of concern to painters who will be staining the canvas with thin transparent layers where the slight discolouration might show through. And even though it can act as size, primer and ground other artists only use acrylic primer for the sizing and priming qualities but do not like the ground it provides, finding it too absorbent or too toothy, so they might mix some matt medium into the ground before applying or apply an oil ground on top of the acrylic primer after it’s dry. If you want a more absorbent surface you can mix modelling paste or extra whiting or other chalk or marble dust into the ground before applying it. You can mix an acrylic texture paste such as pumice paste in with the acrylic primer/ground before applying it if you want more tooth, or mix in pumice powder. If you have a stretched canvas that is universally primed (meaning you can paint on it with oil or acrylic paint), there is an acrylic primer on it that has sized the surface, so you can add any custom ground on top of the provided priming, to give you the exact surface you want to paint on. Some examples are oil ground, absorbent ground, pastel primer and many other possibilities.

An acrylic ground is the usual choice for preparing watercolour paper for acrylic paint or oil paint. It will add body to the support so that the amount of paint isn’t heavier than the amount of paper. Two coats is usually enough to seal the paper from the damaging effects of the oil for oil painting.




Acrylic primer/grounds -left to right: Golden heavy body acrylic paint Titanium White, AV Gesso Acrylic Primer, R&F Encaustic Gesso, Daler-Rowney Gesso Primer white, Winsor & Newton Artists Acrylic White Gesso, Liquitex Super Heavy Gesso, Roberson Acrylic Primer.



Every brand of acrylic primer has a different formula and some are better at being sanded smooth, some are less or more absorbent, some crack more when applied thickly or dried too quickly, some are not very flexible and state they are for rigid surfaces only, not stretched canvas or paper. Some brands make a variety of types, calling their less absorbent version ‘acrylic primer’ and calling their more absorbent version with more whiting (chalk) added ‘acrylic gesso’. Michael Harding makes one for oil painters that is non-absorbent. All acrylic primer/grounds can be tinted with a little acrylic paint before applying, if desired. Be aware that cheap primers (which we don’t sell) may not adhere very well to the support and may peel off later. Just to note: Acrylic Primer is called ‘acrylic’ because it is made of acrylic, not because it is for use just with acrylic paint.




A wide soft brush alone or followed by a large palette knife used like a squeegee, are great for applying grounds.



Applying a Size or Ground

A size is generally brushed on with a wide, flat brush or scraped on with a squeegee, palette knife or spatula. Scraping may help to fill the holes of a more loosely woven canvas. The first coat is worked into the fibres a bit. You apply 2 – 4 coats to seal the support from oil paint, or one to seal it from Support Induced Discolouration (SID). Each coat can be applied as soon as the previous is touch dry.

A ground can be applied with a brush, palette knife or paint roller. Each coat can be applied as soon as the previous is touch dry.




Testing different acrylic primers on linen and cotton canvas. Shown here wet.


Acrylic ground differs in thickness, opacity and grittiness, depending on the manufacturer. It is usually too thick to use straight out of the bucket and should be diluted with a small amount of water (up to 10%) until it is the consistency of heavy cream. Most primers have instructions that advise you that it is better to apply three thin coats rather than one thick coat – a very thick coat may crack as it dries. For oil painting, the first coat is often scrubbed into the weave of the raw canvas in circular motions to be sure that it is well sealed, since it is acting as a size as well as a ground. Then subsequent coats are applied in alternating directions across the canvas. An acrylic ground can be lightly sanded between layers if you wish it to be particularly smooth (take care to wear a dust mask).


Three ways to prepare your surface for oil painting

On linen, cotton or paper you can seal the surface to prevent penetration from oil paint and have a good ground to paint on, in one of these three ways:

1. Apply a clear size and ground, usually 2 to 4 coats of acrylic medium or rabbit skin glue, which will be your final surface. Takes one day to dry. You would do this if you wanted to see the surface because you will be leaving part of the brown linen to show, for example, or you like painting on a mid-tone. The acrylic medium provides a smooth surface that is not very absorbent, so you can wipe back oil paint as part of the painting process. The RSG provides a smooth surface that the paint settles into.

2. Apply a clear size to seal the fibres and then an oil ground. Remember, an oil ground is essentially oil paint, so it will take the time of oil paint to dry and will need to cure for a week or two. You would use this surface if you like the feeling of the brush on an oil-primed surface. There is just the right amount of brush drag and it is not as absorbent as acrylic primer so you may have fewer sunk-in areas where the oil has absorbed out of the paint into the primer, and it is white if you want that surface to reflect back through your transparent colours.

3. Acrylic primer/ground, usually a first coat thinned with 10% water, then two more coats, applied when the previous is touch dry, so all coats can be done in one day. Then wait 3 days for it to cure, so the oil will bond to it properly. You would use this if you like an absorbent surface, you can’t wait for one to two weeks for an oil ground to cure, prefer the ease of one product that does the job of both sizing and ground, and that is water-mixable. If you wish to use an acrylic primer but would like the “faster” feel of oil primer (more slippery, less absorbent, slower drying), then you can wipe linseed oil over the whole surface and wipe it back off, leaving the barest film and then paint onto this, wet or dry. The absorbency of acrylic primers varies a lot and Michael Harding makes one for oil painters that is non-absorbent.

If you wish to use genuine gesso for oil painting you will need to apply it to a rigid panel.




Toning a ground with a mixture of burnt sienna oil paint and turpentine, and then wiping most of it off with a rag allows the white of the canvas to show through the thin layer and give a little glow to the colour, as well as insuring it will be dry enough to paint on the next day.


Toning the Ground

Painting on a white canvas can cause you to paint in colours lighter or brighter than you intended, so you then need to adjust your colours after you have more of the white covered and you can see the actual colours better next to each other. This is one of the reasons that many painters prefer a wooden palette. To avoid mis-judging the colours or values on the white, some artists apply a middle value on the whole canvas before they start, this toning of the canvas also prevents unwanted bits of white canvas showing through your brushwork and you can leave bits of the tone colour showing for added liveliness. Sometimes a middle value, toned ground is used to allow the painter to only need to add the light colours and the dark colours, leaving the toned part unpainted as the middle value. In keeping with the fat-over-lean rule, toning is usually done with a lean mixture, the leanest mixture of the painting. Usually a thin wash of just a bit of paint in a lot of solvent is wiped over the entire surface with a rag. A warm, middle tone like burnt sienna might be used to add warmth to a painting that will be composed of mostly cool colours. Using burnt umber would be toning with a more neutral colour. It is traditional to use earth colours because they dry faster than other colours and you can usually paint on the canvas the next day. I often scrape leftover paint off my palette and use it to tone a ground, which doesn’t dry as fast because it often has some slow drying colours in it like titanium white. You can also experiment with applying black and coloured acrylic primer/grounds.


An Acrylic Exception – Staining Canvas with Acrylics and Flow Release

If you want to use a staining method with acrylics on raw canvas there is another way to get around the beading up that you would get with a watery acrylic mixture on raw canvas. Skip both sizing and a ground and instead add a small amount of flow release wetting agent to the paint which decreases the surface tension and allows it to flow and be absorbed by the raw canvas. Because acrylic doesn’t contain the oil that damages the cellulose fibres you don’t need to be concerned with protecting the canvas from the acrylic, so sizing isn’t necessary. Please note that these are concentrates and you will add 20 parts water to create flow release water that you then use with your paints.

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