Support Induced Discoloration or SID describes a phenomenon that can occur when the acrylic appears to change in color upon drying. It usually takes on a yellow, orange or brown tint, due to impurities in the substrate being drawn up into the acrylic film. The discoloration occurs while the paint or medium is drying and curing and should not continue or happen after the film is cured. This can be found in supports including woods, hardboards, particle boards and some canvas and linen supports. These impurities can include glues, resins, sizing and any soluble materials in the substrates. SID is only applicable when painting with acrylic paints and mediums and most noticeable in thicker applications of clear or translucent mediums and gels and in opaque and semi-opaque pastes. It can affect some paint colors as well.
In 1989, GOLDEN partnered with the Art Conservation Department at Buffalo State College to try to understand what was causing a fast yellowing or browning of a matte acrylic film on canvas. Through an error in the testing of a small corner of acrylic gel becoming detached from the canvas and remaining clear, it was determined that the substrate was causing the discoloration and not just the acrylic film changing color on its own. The term Support Induced Discoloration or SID for short was then coined. (1)
Our recommendation to block SID on these substrates has historically been GAC 100, but recent testing has shown that our Gloss Medium is more effective and we have changed our recommendation to reflect that. As you can see in the image below, applying 2 coats of Gloss Medium directly on the support before applying any other product, is most effective in blocking SID from the thick application of Regular Gel Gloss. In the case of OPEN Gel Gloss, which has a very slow dry and curing time, the wet product lingering on the surface allows more impurities to be drawn into the gel and the Gloss Medium is less effective. This is a “worst case scenario” test, as we only recommend painting with OPEN products thinly. It should also be noted that Gloss Medium and GAC 100 do not seal wood, they really just serve to block SID. Washing the canvas before painting can be somewhat effective, but usually this results in a wrinkled fabric and it can be hard to rid of all of the wrinkles, even when stretching. Washing linen was less effective to stop the Support Induced Discoloration. (2)
From left to right: No sizing and Gesso only, One Coat of GAC 100 then Gesso, Two Coats of GAC 100 then Gesso, One Coat of Gloss Medium then Gesso, Two Coats of Gloss Medium then Gesso
More information on Support Induced Discoloration can be found here in the video Support Induced Discoloration (SID), what is it? and how to minimize it: https://vimeo.com/273304693 and on the goldenpaints.com website here: https://www.goldenpaints.com/technicalinfo/technicalinfo_prepsupp or by contacting GOLDEN at 800-959-6543 or by emailing us at [email protected].
(1) , (2) Hamm, J, Gavett, B, Golden, M, et al., “The discoloration of Acrylic Dispersion Media”, Saving the Twentieth Century: The Conservation of Modern Materials, Canadian Conservation Institute, 1991, 381-392
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Are you recommending the gloss medium be used over acrylic primed canvases?
Great question. We are currently doing tests to see if the Gloss Medium would work to block SID on top of Gesso and will revise the article to include the results. We suspect that there would be very little SID with a pre-primed canvas in general. We see more if this occurring with wood and particle board panels.
What about canvas that has been primed with gesso?
What can you use to prevent SID on a pre primed canvas if gloss medium only works on non primed surfaces? Can you mix gloss medium with white gesso and put a coat before painting to prevent SID, or put a layer of gloss medium on preprinted canvas and add another coat of gesso over the top?
Using the technique described above to use Gloss Medium as a primer with wood, is there a sealer you recommend for wood, such as birch board, etc? Or can one use any product like Kilz Max (Ph 5-8) or something else? I am embarking on a project using large custom cut, high quality birch boards from a local wood working shop & need to figure out what my best choice is for a sealer. After I seal, I could cover with Gloss Medium. My next steps are an acrylic layer or two, then oil & cold wax. Thanks for any pointers.
Hi Lisa,
If blocking moisture to the wood is your concern, you could prime with an alkyd primer. Support Induced Discoloration is only an issue with acrylic paints, so there would not be a need to use the Gloss Medium on the wood if painting in oils. We generally recommend either 3 coats of Gesso or Fluid Matte Medium (if you would like a Clear Gesso) directly on panel, linen or canvas to block oil penetration before painting with oils. Here is an article about preparing panels for a life outdoors that talks about alkyd primers on wood panels: https://justpaint.org/preparing-panels-for-a-life-outdoors/
We hope this is helpful and if you have any other questions, feel free to email us at [email protected]
I have often used a gloss medium sealing coat (usually tinted) over canvas that has only one coat of gesso. I haven’t noticed discoloration in the sealing coat. Since I usually paint the whole canvas I don’t worry. Even if some discoloration occurred it doesn’t seem to migrate into upper layers.
Thank you Lloyd for sharing your experiences. That is very helpful!
Best,
Stacy
How much raw canvas area can be covered with two coats using a 128 oz. jug-o-medium?
Hi Lloyd,
It is really dependent upon the absorbency of your surface and how thickly it is applied. A very rough estimation would be around 300 square foot per gallon, but you can do a more precise estimation by testing yourself. The details can be found in this article: https://justpaint.org/estimating-square-foot-coverage-for-products/
Best,
Stacy
Hi! What experiments have been done for creating a product that’ll pulling these impurities out before even starting with gloss medium/gesso? I have an open gel jar that I don’t expect to use simply because of how subject it is to discoloration, which I’ve taken to laying out on a board before I begin. I spray it repeatedly, then after a while scrape the whole layer off and trash it. I’ve attempted a few experiments in the last year (but have unfortunately misplaced the boards) and it seems to decrease the amount of impurities that CAN come up later. I wonder if Golden has considered creating a product that is literally just for sitting on the board for a bit and then for being wiped away?
Support Induced Discoloration only occurs upon drying, so should not cause discoloration once the products have dried and should not cause discoloration later. Two coats of Gloss Medium on the substrate before painting proved to be a sufficient SID blocker in our tests. After coating the surface with the Gloss Medium and left to dry a few days, the OPEN Medium should be fine to use and not discolor later from SID. Your idea is an interesting one, but we’re not sure how many applications it would take to remove enough of the impurities in the surface, or if it is even possible to remove all before starting. If you are working on panel, you might consider using Medium Density Overlay (MDO) or Birch Plywood, as these tend to be cleaner wood alternatives that shown to have less issues with SID in our testing. Here is an article about MDO you may find useful: https://justpaint.org/medium-density-overlay/
If you heave more questions or would like to discuss further, please feel free to email us at [email protected].
Dear Stacy,
How do we seal a wood for oil painting since GAC 100 is not a sealer?
Hi Jasmine,
Support Induced Discoloration is not a concern for oil paintings. GAC 100 can be used on wood prior to oil painting and while it is not a sealer, the wood does not need to be sealed for oil painting, but it works to block oil penetration into the wood and keeps it in your paint. Two to three coats of almost any acrylic can block oil penetration, so you can use two coats of GAC 100 or another acrylic medium and one coat of Gesso or simply three coats of Gesso if you want a white ground. If you would like a clear gesso, you could use three coats of our Fluid Matte Medium. This article about preparing a cancas for oil painting may be helpful: https://justpaint.org/preparing-a-canvas-for-oil-painting/
We hope this is helpful and if you have more questions, feel free to contact us at [email protected].
Hi thanks for sharing this information!
Can GAC-100 (or Fluid matte medium) be tinted white (with white acrylic paint). If so is there a ratio so GAC 100 retains it’s properties?
Thank you!
If you would like to tint GAC 100 for blocking SID and your substrate is prone to SID, the white will likely discolor upon drying. The Fluid Matte Medium does not block SID. If you would just like to mix white acrylic in with the GAC 100 or Fluid Matte Medium for painting, that should be fine. There is no ratio limit for mixing acrylic color into acrylic mediums. If you have further questions, feel free to contact us at [email protected].
On wood an acrylicbclear sealer is suppose to be ok?
Hello Lyne,
Not all acrylic products block SID equally, and we have not tested the product you mentioned. The most effective product we have have is Gloss Medium. Two coats should be sufficient to block discoloration.
Hi! its really necessary to apply gesso after the gloss medium? I mean, does gesso has any SID blocking properties or it’s applied only for the “tooth” in this case?
The Gloss Medium should block the Support Induced Discoloration, so if you are painting with acrylics, you don’t need to apply Gesso on top of the Gloss Medium unless you prefer a white and toothy ground.
Using titanium white acrylic straight from the tube instead of gel medium wouldn’t be the same?
If your ground has a lot of impurities and you do not use the Gloss Medium to block SID and apply Titanium White Acrylic directly on the Ground or over Gesso, you could see some discoloration in the Titanium White Acrylic when it dries. We hope this is helpful and if you have further questions, please feel free to contact us at [email protected]
Thanks a lot for responding!
Yes I mean if its possible to use any Pigmented Acrylic as a replacement to Gloss Medium.
Once the Pigmented Acrylic dries and the discoloration becomes visible, would the discoloration affect subsecuent acrylic layers?
We tested most of our products and the Gloss Medium, aside from GAC 100, was the only medium that worked to block SID, so the paint itself will not block it. If the paint were to discolor, it’s hard to say if that would then block SID. It would really depend upon how dirty the surface or how thin the first layer was to know if it would travel through multiple layers. We have tested applying the Gloss Medium on top of Gesso (which by itself does not block SID) and the Gloss Medium did block SID from subsequent layers, but the Gloss Medium discolors. We hope this helps to clarify.
Can you tell me if I can use a generic wood primer to seal the surface? Or does this only make the paint stick to the wood and I’d still need to apply gloss medium? Thanks.
Hello Kay. You could use a commercial wood primer as long as it is designed to accept acrylics or latex paints. Gloss Medium is the best product we make to block SID from occurring, but there are other stain blocking primers out there. Gloss Medium is basically paint without pigment, so paints will stick to the wood without a primer. The Gloss Medium Helps block SID, but a ground like Gesso or primer can provide a nice white ground if desired. Let us know if you have more questions. [email protected]
Scott
Hello, thank you for your suggestion. Unfortunately, gesso fails to block Support Induced Discoloration and we have seen the discoloration travel through multiple layers of gesso. However, the discoloration tends to be less of an issue when the gesso is covered with opaque paints and/or colors not impacted by the addition of the brownish yellow. Support Induced Discoloration can have the most dramatic impact when glazing with transparent layers or using colors which might change dramatically through the addition of a brownish yellow — violets and blues and whites, for example.