MITRA Forum Question Details

ApproveRejectUn-ApproveThanks, 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.

ApproveRejectUn-ApproveI 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.

ApproveRejectUn-ApproveHi 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.

ApproveRejectUn-ApproveThanks, 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!

ApproveRejectUn-ApproveYes 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

ApproveRejectUn-ApproveDear 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?

ApproveRejectUn-ApproveI 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?

ApproveRejectUn-ApproveRegarding 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

ApproveRejectUn-ApproveThanks, 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.

ApproveRejectUn-ApproveOn 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

ApproveRejectUn-ApproveYes 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

ApproveRejectUn-ApproveAnother 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.

ApproveRejectUn-ApproveYou 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.

ApproveRejectUn-ApproveSo there harm from leaving unsealed then?

ApproveRejectUn-ApproveIt 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

ApproveRejectUn-ApproveThanks, 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!