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  • The best practice in prepping a stretched canvas for oil paintApproveRejectUn-ApproveSubscribeUn-Unsubscribe
    Question asked 2016-11-10 09:43:46 ... Most recent comment 2016-11-10 11:59:00
    Art Conservation Topics Flexible Supports Sizes and Adhesives
    Question
    ristin DeGhetaldi, I am researching sizing of canvas for stretched supports. I have tried both Gamblin PVA and GAC100 for preventing the leaching of oil to the canvas, to prevent rotting. I prefer using the Gamblin PVA, because it seems to soak into the canvas better, and therefore is easier to use (also, it is less cost prohibitive). However, I am still concerned about too much flexibility with either of these PVA sizings when used with acrylic gesso and stretched canvas. The fully cured oil paint will be more rigid than its substrate, which could lead to cracking, long term. To do it better, and get more compatible flexibility, I am thinking that adding GAC 400 might be a good option. This would make the substrate stiffer and of similar rigidity to the fully cured oil paint. Am I right here?

    If so, what would be the best order of operation? I am guessing a layer of Gamblin PVA on the front then back, before stretching, then a layer of GAC 400 on the front, after stretching. Then I would wait a day before gessoing twice with high quality acrylic Gesso, like Golden. Then, I would wait 3 or 4 days before painting. I would appreciate your opinion. Thanks!
Answers and Comments
  • EditDeleteModerator AnswerHi there....I am sure one of our moderators who is familiar with GAC products will respond soon. But in the meantime feel free to consult our documents in the Resources section....particularly the "Flexible Supports" regarding stretching techniques and "Adhesives and Sizes" as well (do not forget to scroll down to the bottom of the last document as there are many helpful links there).
    Baade, Brian
    2016-11-10 12:04:08
  • EditDeleteModerator AnswerHi - The first thing I want to point out is that GAC 400 only works if applied directly to raw natural fibers and so should NEVER be applied to a canvas after already sizing it with PVA or any other material. The reason is that GAC 400 is not meant to be used as a continuous film, where it would prove to be too brittle, but really as a thin penetrating coating of the fibers that stiffens them. Also, if the GAC 100 feels too thick or is not penetrating into the canvas enough, it can be thinned with some water (up to 1:1), but then of course the resulting film will be thinner and multiple coats might be needed to get the same amount of oil blocking. In terms of using acrylic sizes for preparing canvasses, I would recommend reading through the following article where we summarizes a lot of the testing we did using different combinations of materials and looking at how stiff and effective at oil blocking they were. These tests included PVA size and Rabbit Skin Glue, so were not restricted to acrylic alone: http://www.justpaint.org/preparing-a-canvas-for-oil-painting/ In terms of stiffness, you are correct that the stiffer you can make the canvas the better, and that using GAC 400 is definitely helpful. in this regard. And, as you will see in the above article, if you have a studio or workspace where the temperature is reliably at 70F or above, another option is simply to skip the GAC 400/GAC 100 combination and use just two coats of GAC 200 followed by either an acrylic gesso or an oil ground. And finally, if able to coat the canvas prior to stretching, another ideal system would be to use GAC 400 on the back side, stretch, then apply a coat of GAC 400 to the front, and then simply apply 3 coats of acrylic gesso as we know 3 coats of a high quality one will block oil penetration and would be stiffer than, say, GAC 100 followed by a couple of coats. Finally, keep in mind that the canvas will remain the most flexible material in the system, even after all of these steps, so using a panel will always be the most stable option, or when possible, mounting the canvas to a panel afterwards.
    Sands, Sarah
    2016-11-10 14:33:17
  • ApproveRejectUn-ApproveUser CommentThanks, Sarah! I did read your suggested link. The quality of information was fantastic. I have some questions. If I size with GAC200, two coats, how long would I have to leave the heating at 72 degrees for proper curing? How long between coats? How many coats of Golden Acrylic Gesso would be ideal for glazing? I have noticed that a medium openness is ideal for holding a glaze. What would be the ideal procedure for mounting canvas to panel? Thanks so very much.
    2016-11-10 15:02:37
  • ApproveRejectUn-ApproveUser CommentI have learned that there is no sizing on virtually any premanufactured canvases in Canada to prevent the oil from doing damage. This has spawned my research. If any one knows of manufacturers that properly size their canvas before gessoing, please share this information with us all.
    2016-11-10 15:05:37
  • EditDeleteModerator AnswerHi to answer your first comment....Sarah can answer with more specifics regarding sizing using acrylic gesso for glazing. Information regarding how to adhere your canvas to a rigid support, please refer to our "Rigid Supports" document as there are instructions outlining various procedures for doing so. As for your second comment there may well be a few brands of commercially available canvas that are sized (it is difficult to confirm this without analytical testing and/or obtaining confirmation from a manufacturer), but it sounds as if for the most part the many brands you have tested have not been to your liking in terms of being able to prevent "strike-through." We will reach out to a couple industry reps who sell commercially primed canvases who may be able to provide you with additional information regarding this topic....it may also be that you could purchase a higher quality commercially pre-primed canvas and simply apply an additional layer of acrylic dispersion ground ("gesso") as well.
    Kristin deGhetaldi (CAS)
    2016-11-10 16:49:05
  • ApproveRejectUn-ApproveUser CommentHi Kathy! Happy to help further. The GAC 200 only needs to dry while at 70F, so once it appears clear and not cool or clammy to the touch, you are fine. Could easily be a matter of a few hours. As for time between coats, again, once clear and touch dry, you are good to go. The first coat usually dries very quickly because of the absorbency of the canvas, so I could see doing the second coat 1-2 hrs after the first, and then letting that sit for another 3-4 hours to be safe before turning down the heat. As for the ideal number of coats of acrylic gesso for glazing....great question and would think it might vary depending on each person's taste. I would start at 2 coats as that will still hold onto the texture of the canvas, and then if you want something smoother, add a third. And finally, for mounting canvas to panel, you have several options. Having a canvas removable for conservation purposes is always best, so if willing to explore the use of BEVA, which is a heat activated adhesive used frequently in conservation, you can purchase that as a brush-on gel or as a film. We purchase ours from Talas, which you can find at Talasonline.com (put BEVA in their search box)and you should also read the Resource Page for Adhesives and Sizes that you can find here https://www.artcons.udel.edu/mitra/resources as that has a section on BEVA and even its use for mounting canvas to panel. If not concerned with reversibility, you can certainly use an acrylic gel or medium as an adhesive. You can see a short video where we show the use of Soft Gel for adhering paper to a panel, but the process for canvas would be similar: https://www.youtube.com/watch?v=ggB1veQmwNU&t=1s One concern I would have in either of these cases is that tyou would be working with a finished painting, rather than simply adhering unpainted canvas to a panel, and so there might be risks to damage or deformation of texture in the painting from any application of pressure and heat, so hopefully one of the conservators might be able to chime in with a best practice in that case.
    2016-11-10 17:13:20
  • ApproveRejectUn-ApproveUser CommentThanks, Sarah. I like the simplicity of the the double coat of GAC 200, the complete seal to the canvas (even if the gesso were to get a fissure, and leak, the canvas would be protected), and the stiffness is excellent, yet it isn't brittle and it is not hygroscopic. So it sounds like it would be as stable a ground as canvas can give you. I will give it a dry-run before recommending and teaching it to my colleagues. I will also read up on mounting canvas on panel and recommend this even sounder practice to them as well. At this point after just reading your reply, I think the BEVA glue sounds good, as well as doing the gluing before painting. Your help, and this forum is just great. Thank you so much for doing this for our art community!
    2016-11-10 23:16:43
  • ApproveRejectUn-ApproveUser CommentYes as Sarah stated it is ideal to first attach the canvas to a panel BEFORE painting....but we do address this in the Rigid Support document listed on our Resources page (specifically how one could go about attaching an already completed painting). Kristin deGhetaldi
    2016-11-10 23:27:09
  • ApproveRejectUn-ApproveUser CommentDear Sarah, What is the process using 2 coats GAC 200 then Golden Acrylic Gesso? Best Guess: 1) Coat one side of canvas with GAC 200 and let cure at 72 degrees 2) Stretch canvas, with coated side towards the back 3) Coat the front side on stretched canvas with GAC 200 and let cure at 72 degrees 4) The following day, prime with Golden Acrylic Gesso and sand when dry 5) Coat with another layer of Golden Acrylic Gesso 6) Wait 3 or four days before painting What is the process using GAC 400 then GAC 100 or Gamblin PVA, then Golden Acrylic Gesso? Best Guess: 1) Stretch canvas 2) Coat front side with GAC 400 and let dry 3) Re-coat front side with GAC 100 or Gamblin PVA and let dry 4) Recoat front side with GAC 100 or Gamblin PVA and let dry 5) The following day, gesso with Golden Acrylic Gesso and sand when dry 6) Coat with another layer of Golden Acrylic Gesso 7) Wait 3 or four days before painting The other process you mentioned was: 1) Coat backside of canvas with GAC 400 and let dry 2) Stretch canvas 3) Coat front-side of canvas with GAC 400 and let dry 4) The following day, apply three coats of Golden Acrylic Gesso, sanding in between 5) Wait 3 or four days before painting Do I have the processes and products right for these three options? Of the three basic processes, which would you recommend the most? Also, could the second process, (with GAC 400 and 100), could the process be enhanced by also first coating the back-side of the canvas with GAC 100 or Gamblin PVA so that moisture or oil from fingers could not seep in?
    2016-11-11 14:05:46
  • ApproveRejectUn-ApproveUser CommentI have been reading suggested materials about canvas mounted to panels. The BEVA glue for process for mounting would be great because it is reversible and therefore lends itself to making works repairable for future generations. The problems however are that it has toxic fumes, and is very complex process. It is likely time consuming and expensive, which makes it rather unviable for a professional artist who already operates in a time deficit for projected income. I have another idea. Would there be any problems associated with prepping canvas in similar ways to those suggested for stretching on flexible supports, but instead of using stretcher bars for supports, using a pre-manufactured hardwood plywood panel – one of those with wooden bar supports on the back sides and cross bars in the middle? Could I tack canvas to one of these, stretching it the same way that I would on stretcher bars, leaving the back unglued to the panel? This would only have the mechanical attachments of the tacks so would allow easy access to the painting by art conservationists, if needed sometime in the future. It would be rigid and stop flexing of the canvas. It would allow the substrate materials to have one less material (mounting glue) to counter against in movement. What do you think?
    2016-11-11 14:37:57
  • ApproveRejectUn-ApproveUser CommentRegarding your question relating to BEVA-mounting canvases...you do not necessarily have to use BEVA as you will see in our document "Rigid Supports"....it is possible to also use acrylic gel medium or even an appropriate PVA adhesive. Certainly the method you propose above is fine as well as it is simply another reversible option. Of course there may be some slight mechanical-related damage that is localized around where the painting is tacked to the wooden support but in the end this may be a minor drawback. Kristin deGhetaldi
    2016-11-11 15:13:38
  • ApproveRejectUn-ApproveUser CommentThanks, Kristin. Yes, I read about using PVA or acrylic gel for glue. But there was an advantage to the BEVA in that it was not a permanent mounting. So if I can achieve the same end for no health or time compromise, I will go with the canvas stretched over the panel and tacked. My colleagues that I am teaching will like it to. It is easy and is not much of a change from a stretched canvas over flexible support. But it is so much better. Look forward to confirmation of the three processes of canvas prep we have talked about and which one you prefer.
    2016-11-11 16:16:02
  • ApproveRejectUn-ApproveUser CommentOn the question of the different ways to size and prime a canvas, let me start by admitting that all the options you lay out sound viable and which one is 'best' might really come down to which you find easiest and which results meet your needs the most. If you have the time and wherewithal, it might be interesting to prepare three 16"x20" or similar sized canvases using each of the three methods and comparing them. But also let me make a few comments on each process below:

    1) We have, to be honest, never tried applying GAC 200 to each side. In the article what we did was apply GAC 200 to the front of a stretched canvas, then applied a second coat once that was dry (usually 3-4 hours later) That said, it might be interesting to try your system to see if it is equally effective or perhaps more so. But certainly if wanting to follow what we did, then two coats to the front, wait a day, then apply acrylic gesso layers, then - as you say - 3-4 days before painting. Also I notice that you keep quoting the temperature as 72F. Just know that we set it at 70F with built in wiggle room already since, speaking strictly, the min. temperature was closer to 65F but we figured that with thermostat inaccuracies and uneven heating in a room, 70F seemed safer.

    2) Yep, this sounds fine. Minor note - we have never tried applying PVA on top of GAC 400 but see absolutely no reason why this should be an issue. Just wanted to be honest about what we have or have not tested ourselves.

    3) This sounds perfect. If coating the backside, then stretching, becomes a hassle, using one coat GAC 400 to the front followed by three coats of acrylic gesso would work as well.

    Which would I suggest? As I said, I think it is really a personal choice. All are fine. I personally like the straightforwardness of the first and third systems, or even the very simple 4 coats of acrylic gesso to the front, which is fairly stiff as well. But see what you think. Each will give subtle differences in their final surface and feel.

    Finally, on the question of coating the back of a canvas to protect it from finger oils....it feels like overkill to me but I do not necessarily see a downside. Of everything you can do to protect the back of a canvas, I feel by far a backing board is ideal. It helps moderate any shifts in the humidity, dampens vibrations when moving or shipping, and protect from dust or dirt.....and finger oils! I find this document from the Canadian Conservation Institute easy to follow

    Backing Boards for Paintings on Canvas - CCI Notes 10/10

    but others here might be able to point to other resources as well.

    Hope that helps!

    Sarah Sands
    2016-11-11 16:21:18
  • ApproveRejectUn-ApproveUser CommentYes BEVA is reversible....but realize so too are most PVA adhesives and acrylic mediums in certain instances....otherwise we would not recommend them. Kristin deGhetaldi
    2016-11-11 17:45:37
  • ApproveRejectUn-ApproveUser CommentAnother question: If I stretch a canvas sized on the back with either Gamblin PVA, GAC 200, GAC 100 or GAC 400, over a cradled wood panel, should I also size the panel and cradle, or should I leave it to breath? Thanks.
    2016-11-12 10:52:08
  • ApproveRejectUn-ApproveUser CommentYou certainly can....there is nothing wrong with sealing the wooden panel/cradle. You can use a variety of materials to do this (PVAs and acrylic mediums are not well-suited for this purpose)....Paraloid B72 is a resin that many conservators use (and can be purchased at Kremer and Talas) but you can also choose to use a high grade shellac or a solvent-born polyurethane.
    2016-11-12 11:27:21
  • ApproveRejectUn-ApproveUser CommentSo there harm from leaving unsealed then?
    2016-11-12 13:01:40
  • ApproveRejectUn-ApproveUser CommentIt is probably better to seal than not to seal but we do have examples of very, very old paintings being stretched around panels that have survived just fine. Wood can off-gas things like formic acid and other potentially harmful components but since you are sizing BOTH the back and the front of your canvas with PVA or GAC you will likely be fine on this front. Kristin deGhetaldi
    2016-11-12 13:20:23
  • ApproveRejectUn-ApproveUser CommentThanks, Kristen. Think I can move forward with this now! Look forward to talking about other stuff as it arises. So good to be able to get reliable info!
    2016-11-12 13:37:24
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