Gesso vs Primer vs Grounds vs Size - What's the Difference?

Tuesday, May 18, 2010

Preparing to paint - Gesso, Primer, Grounds, Size
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.

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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.

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25 thoughts

  1. Very informative, thank you. still a bit confused but I will re-read. =)

  2. Must oil gesso be used over RSG or is acrylic gesso ok?

  3. From everything I've read, you should avoid RSG like the plague as it causes more issues than it solves because it has a tendency to draw in water. So I don't know why you'd use it when there are better options out there. As for acrylic gesso over RSG, I don't think RSG is a good base for protecting the paint from the canvas, although I'm making the assumption that you could use it this way. This is really a question for the tech boffins in the AMIEN forums:

    1. keeps sending me to an E Cig site. Maybe the link is old, and if it is, do you know the current address for The Art Materials Information and Education Network?

    2. From here:
      Why was the AMIEN (Art Materials Information and Education Network) forum closed and are there plans to resurrect it?

      Mark Gottsegen was the administrator, and he died in 2013. I can only assume that is the reason the forum closed.

  4. Hi Fiona and friends

    WOndering if you could help me with a query i had..?

    I'm planning to make quite a large painting on a large square of free canvas rolled out on the ground.

    I'm using a roll of preprimed basic canvas..
    So my query is as i'd like to thicken it out with extra primer of some sort, making it stronger yet quiet malleable to roll.

    I would like to make this light enough canvas almost weighed and aged by the application kind of like the feeling of an old wax jacket. It doesn't necessarily have to be a oil finish, as im not sure if im going to be using oil or acrylic yet.

    Im planning to paint on the ground in public and it will take a lot time to finish so ill be rolling it up and out many times, taking it out and bring home everyday, therefore these extras layers will make the large sheet of canvas just more substantial as in durable also.

    If anyone gets the kind of feel for how i want to process this pretty flimsy large rectangle of canvas, before i take it on to the street then Please let me know what i should do, or any suggestions would be appreciated greatly thanks.

    Regards from Berlin,

    Niall D.

  5. Hey Fiona! Thanks for the great clarification of all this confusing stuff. Did I understand correctly that I can paint on a panel primed only with pva sizing and no gesso? Does that apply to oil painting or just acrylic? I'm about to start an oil painting on a HUGE panel and am paranoid about preparing it correctly since it's such a huge financial and time commitment.
    Erin Hardin

  6. Hi Erin,
    not quite. You need a suitable barrier (size) between the panel and the paint and so you need to know whether you will be using acrylic or oils to choose the best size for the job. For oils "it is now recommended to use a pH neutral PVA or any acrylic medium whose manufacturer recommends it as a size." For acrylic "the best choice is acrylic medium whose manufacturer recommends it as a size that can block SID - a blocking size."

    Priming - adding tooth for the paint according to personal preference - is optional.

  7. Why can't you paint oils directly over rsg? They contain some of the same ingredients as the oil based primers.

  8. Hi Anon,

    sorry, I don't recall the reasons why oil painting directly on RSG is not advisable. I'm sure the AMIEN forums can enlighten you. What I clearly recall is that RSG is a major problem (detailed in this article) and should be avoided completely, so the question seems quite academic really.

  9. Hi, i have plywood as a surface ( with support on back), can i use GAC 100 and then use acrylic gesso on top of this? michael harding seems to suggest not?
    i intend to use oil paint then.

  10. Ian,
    "underneath oil paintings, it is now recommended to use a pH neutral PVA or any acrylic medium whose manufacturer recommends it as a size." Does GAC100 fit the criteria?

    Acrylic gesso is meant to be fine under oil paint with caveats "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....
    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."

  11. Thank you,
    I had no idea what any of it was so thank you very much

  12. I must protest against your suggestion that using an acrylic ground under oil paint is a better solution than using Rabbit Skin Glue.

    What you are trading is the alleged cracking induced by RSG for the wholesale delamination of the oil paint from the acrylic base some 50-100 years hence. Artists would hate much more having their entire painting peel off in a sheet rather than having cracking. For that reason, the Smithsonian entirely forbids the use of an acrylic base underneath oil paint. Simply put--oil paint does not stick to plastic.

    One problem is that many companies make their bread and butter by selling pre-gessoed canvas (acrylic gesso) for use by oil painters. It is in their financial interest to sidestep the problems they are causing and ones that will cause whole generations of oil painters to have their paintings disappear into flakes as the dried oil paint falls off of the acrylic under support. Because of the time it takes for this to happen, this artistic apocalypse is heading our way.

  13. Tom,
    just a couple of points.
    The alleged cracking caused by RSG is actual cracking. It is known to be a real problem.
    The potential delamination problem is covered in the info if you read to the end. I take my technical painting advice from AMIEN, and I strongly suggest you have a discussion with them about your concerns.

  14. wow, thank you. This was very helpful and to the point. Thank you so much...keep doing this!!

  15. How can I make my own ACRYLIC gesso at home to prime my stretched canvas for my acrylic paintings?
    All the recipes I have found are for traditional gesso.

  16. You can't, as far as I'm aware. Unless you own a plastics/acrylic paint factory.

  17. HI , I recently used an acrylic binder ( made by gerstacker ) to prime a board , really liked the feel of the surface but am not sure if it will last - the painting was done in oils .
    Just wondered if you knew if this was okay

  18. I'm not sure what an acrylic binder is? What I do know is that "any acrylic medium whose manufacturer recommends it as a size" should be ok. The same goes for acrylic grounds, as long as the quality is good. If you're unsure, call the manufacturer and ask. They are the experts on their product. Acrylic is plastic and the technology is complex.

  19. John,
    Please do not accept misinformation from people who assume the makers of acrylic ground have the best interests of oil painters at heart. They do not--they're trying to sell products.

    In fact, the Smithsonian forbids the use of an acrylic ground underneath oil paint. The painting will delaminate eventually. That's why there is a coming apocalypse among contemporary oil painters who have bought pre-made canvasses--with acrylic gesso ground--and then proceeded to paint oils on top of that plastic. News flash--those paintings are all going to delaminate. For impasto, it happens even faster. I saw it happen before my eyes.

  20. Honest question: Is Utrecht lying to me? They tell me on the phone that I can simply gesso over canvas and then its ready for oil paint. I have never seen oil paint "delaminate" from an acrylic ground... and I have been painting this way for 16 + years.

  21. Will,
    any "acrylic medium whose manufacturer recommends it as a size" should be ok for preparing canvas for oil paint.
    My understanding with the acrylic ground under oil paint is that it might become an issue over longer time scales ie 40+ years.

  22. Maor and Murray, Queens university, Canada did a report on some tests involving acrylic primers/grounds under oil that were slightly worrying. Have you seen this?
    Unfortunately they didn't try to minimise the amount of zinc white used in the oils, even if avoiding zinc white in oils may prove very difficult as it seems to show up in small amounts in many colours of paint.

    I have seen older paintings that used acrylic primers showing some flaking but again how much could be due to zinc, aluminium, not removing contaminents with water before starting to use oils who knows. The same goes for painting on the acrylic surface before it has had a chance to completely dry and cure/crosslink after a number of months. Would this help or hinder bonding. The manufacturers don't even seem to comment on this.

    1. Hi Unknown, no I haven't seen the study you refer to. I have just gone and had a quick look for it and found this report on it Yes I agree that avoiding zinc white would be difficult and based on the findings of the study, something we should be doing! Regarding painting on an acrylic surface, the general recommendation is "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." Although I agree the manufacturers don't tend to make this clear. I've never seen one sold with a recommendation to delay painting on the surface for months! I've never found anything to indicate exactly how long the cross linking takes.


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