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OPEN Acrylics, Shellac, and SID

In many ways this article is a continuation from an earlier one,  Waterproof India Inks and Shellac-based Primers, which focused mostly on the alkaline sensitivity of shellac-based india inks when used with acrylics. However, towards the end of the article we briefly looked at  Zinnser’s B-I-N®, a common pigmented shellac primer, to see if it had similar issues. What we discovered was that regular acrylic products, such as GOLDEN GAC 100, appeared to have little to no effect, while ones based on OPEN seemed to reactivate and dissolve the primer quite readily. Clearly this raised some real concerns, especially for anyone using OPEN on top of B-I-N or a similar shellac-based primer.  Not only could the primer itself be dissolved, it seemed, but in the process, shellac’s property of being a strong sealer and stain blocker could become compromised and less effective at protecting from Support Induced Discoloration, or SID. In a nutshell, SID is when moisture from an acrylic application is able to penetrate down into a substrate, allowing surfactants to dissolve any water soluble materials, then pull those materials up into the acrylic film in the process of drying, causing degrees of discoloration. We demonstrate the process in this very short video.

To look into this further we decided to test Zinnser’s B-I-N, as well as GOLDEN GAC 100 and Polymer Medium, to see how effective they were in preventing SID in a worse case scenario – namely painting on top of cheap commercial hardboard panels, which are very prone to causing discoloration. This is particularly true for translucent and thickly applied acrylic gels and mediums, and even more so if they happen to be very slow drying, like OPEN, since this would allow more time for moisture to penetrate into the support. We applied each sealer/primer in one, two, and three coats followed by a thin layer of GOLDEN Gesso in order to provide a uniform white to judge any colorshifts against. We also wanted to compare them to the most common and basic way to prepare the panels: namely, three coats of acrylic Gesso brush-applied and lightly sanded. This would serve as a baseline since we know acrylic Gesso provides little if any protection from SID because it is far too porous, as you can see in Image I below:

OPEN and Regular Gel applied 1/8" and brushstroke thick over three coats of Golden Gesso on hardboard panel.

       Image I:  OPEN and Regular Gel applied 1/8″ thick and as a ‘normal’ brushstroke over three coats of GOLDEN Gesso on hardboard panel.

The two upper discs of Regular and OPEN Gel (Gloss) were applied at a thickness of 1/8″¹ (60 mil), while the two areas directly below show ‘normal’ brushstrokes of the same products. Note just how much more aggressive the OPEN product is in terms of pulling out impurities during the process of drying. If the Regular Gel takes on a translucent, yellowish discoloration, the OPEN product appears darker and much more murky by contrast. However, it is important when evaluating these results, to keep in mind that the test is focused on the blocking ability of the various sealers and is not about the performance of the gels being used on top. Any discoloration of the gels in these tests should not be seen as a fault or failure in them, but rather as pointing to the limitation of the sealer to block SID in a particular circumstance.

Using the above example as a baseline, we can now turn to the other results shown below:

SID Testing of Various Sealers on Hardboard where we looked at how effective GAC 100. Polymer Medium, and BIN were when using OPEN and Regular Gel.

Image 2: SID Testing of Various Sealers on Hardboard. CLICK ON IMAGE to access larger version and zoom in on.

Results: Regular Gel Gloss

Focusing on just the applications with Regular Gel you can see that nearly all of the options performed equally well. Also, there is a consistent improvement as you move from 1 to 3 coats, with the largest gain between the first and second, confirming our usual recommendation of applying a minimum of two coats for blocking SID on substrates like hardboard. Lastly, while it is difficult to show the subtleties in an online image, the Polymer Medium does edge out the other options, in terms of preserving the clarity of the Regular Gel, with B-I-N a very close second, and finally GAC 100 .

Results: OPEN Gel Gloss

Overall the applications using OPEN Gel show a marked increase in discoloration across the board, with a strong difference of degree separating the shellac-based B-I-N from the two acrylic-based mediums. As touched on earlier, a couple of factors are at work here. One is simply the fact that the extended drying time of OPEN Gel allows water and surfactants to penetrate into the wood for a longer period of time, leading to more impurities being dissolved and carried upward into the Gel. The second is that shellac-based products appear to be easily dissolved by the amines contained within OPEN’s unique formulation. Exactly why or how that mechanism works is still something to be investigated, but for now the empirical evidence seems quite strong. Finally, it is worth noting that one can see signs of discoloration even in the thicker ridges of the brushed-on areas, and so the concerns are not simply limited to unusually thick applications.

Protecting from SID on Commercial Hardboard Panels: Best Practices

To protect from SID when using OPEN products on supports like commercial hardboard panels, we recommend the following:

OPEN Products

  • BEST: 3 coats of GOLDEN Polymer Medium followed by one or more coats of GOLDEN Gesso
  • GOOD: 3 coats of GOLDEN GAC 100 followed by one or more coats of GOLDEN Gesso
  • NOT RECOMMENDED: Shellac-based primers or sealers.

Regular Acrylic Products

If using regular acrylic products, more options are available as B-I-N performed quite well in all of the trials with Regular Gel and even had some advantages, such as laying down very smoothly and being extremely easy to sand. Also, being alcohol-based, it would not raise the grain on other types of wood supports. However, as a commercial product, it has not necessarily undergone the type of testing one would want to see for permanent works of art, and formulations can of course change over time. So there is always some level of risk when going outside of traditional art materials. That said, based of the above results, we do think B-I-N and other shellac-based primer/sealers can be useful as one more option.

Acrylic Sealers

  • BEST: 2-3 coats of GOLDEN Polymer Medium followed by one or more coats of GOLDEN Gesso
  • GOOD: 2-3 coats of GOLDEN GAC 100 followed by one or more coats of GOLDEN Gesso

Shellac-Based Sealers

  • BEST: 2-3 coats of Zinnser’s B-I-N followed by one or more coats of GOLDEN Gesso

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¹ 1/8″ is much thicker than we would recommend using OPEN products, as they should ideally be applied thinly and always kept below a 1/16″, or about the thickness of a penny. For more information on this, please see the following Tech Sheets:  OPEN Gels and Mediums, OPEN Acrylic Colors. Regular acrylic gels and mediums, of course, can be applied as thickly as desired.

13 Responses to OPEN Acrylics, Shellac, and SID

  1. Bruce Bundock February 17, 2017 at 2:47 pm #

    I am curious as to any research being done using Golden acrylics on 1. plexiglas, sanded to accept acrylic gesso before painting and 2. sintra as an alternate substrate. Both of these materials seem to me suitable as substrate panels for either directly painting on or mounting canvas to. My current belief is that both are chemically inert, non-acidic. Would appreciate any thoughts on the matter. Thank you. regards, Bruce Bundock Kingston , NY.

    • Sarah Sands February 20, 2017 at 5:17 pm #

      Hi Bruce –

      Thanks for the comment and questions. We have done quite a bit of testing on Plexiglas and other acrylic sheeting and a light scuffing is generally all that is needed to allow for good adhesion. We would share that you should never use any sort of solvent on these types of products, as it can cause damage (Chemical Resistance of Plexiglas ) so to degrease or clean simply use mild soap and water. Also, full adhesion will take a minimum of 72hrs and will continue to strengthen over the course of several weeks, as the last moisture and volatile ingredients leave.

      As for Sintra, the story is a little more complex. We have certainly have excellent adhesion to the surface, and so on that score there are no concerns. However Sintra’s reputation among conservators is mixed, with many supporting its use since it passes many tests for off gassing, while other conservators eye it with a lot of caution and believe much more testing is needed. So that makes it more difficult for us to endorse its use without qualification if only because there seems to be a difference of opinion among experts. That said, if wanting to use it, make sure to coat all the sides with at least acrylic gesso to prevent exposure to UV which is known to cause degradation.

      We hope the above is helpful and if we can do anything else let us know.

      Best –

      Sarah Sands

      • Richard Phipps March 3, 2017 at 3:04 am #

        Hi Sarah,

        Do you know if the situation is similar with polycarbonate (if solvents are also avoided)?

        I am thinking about applying a couple of coats of Polymer Clear Gel and then when dry oil paints with no solvents.

        Thanks,
        Richard

        • Sarah Sands March 7, 2017 at 11:02 am #

          Hi Richard –

          Polycarbonates will not be a source of SID, so that problem in particular is avoided altogether, and certainly none of the acrylic gels or mediums would cause any chemical problems with the polycarbonate. So you should be fine. Just keep in mins that for maximum adhesion for oils we would recommend our Fluid Matte Medium as a form of clear gesso. While oils should have more than adequate adhesion to Polymer Medium, we know that maximum adhesion in any system is achieved when painting on a matte, porous ground.

          Best regards –

          Sarah

          • Richard Phipps March 8, 2017 at 3:10 am #

            Thanks Sarah,

            I was thinking more about using the clear gel as a way to protect against any potential plasticisers leaching through the polycarbonate and paint over time.

            I’ve tried adding clear gesso from W&N to polymer clear gel and it adds a nice tooth for the oils to bond onto whilst remaining pretty transparent.

            Richard

          • Sarah Sands March 8, 2017 at 8:12 am #

            Hi Robert –

            As far as I know and have read there should be no danger of plasticizers leaching out of polycarbonates like Lexan, or similar acrylic sheeting like Plexiglas. But we do think that having a ground that can provide some tooth and absorbency for oil paint to anchor to is a good idea.

            Regards –

            Sarah

  2. David McKay February 18, 2017 at 2:11 pm #

    I am curious to know how traditional rabbit skin glue would work as a sealer under acrylic gesso, or acrylic paint.

    • Sarah Sands February 20, 2017 at 4:36 pm #

      Hi David –

      Thanks for the question. The first question would obviously be what you were hoping the Rabbit Skin Glue (RSG) to protect against. RSG might likely remain an effective oil blocker under acrylic gesso or paint, but we would not expect it to be an effective blocker of Support Induced Discoloration because it is very hygroscopic (or ‘water loving’) and with prolonged contact with moisture or high humidity will cease being a dried film and turn gelatinous. This could in turn allow impurities to pass through. That said, we have not done specific testing of this and so am only offering our best guess. In general RSG has fallen out of favor among conservators and material experts as it has been linked to the cracking of a large percentage of oil paintings and can cause a support to slacken and tighten dramatically in response to changes in humidity. Which by itself is never a good thing.

  3. David McKay February 21, 2017 at 10:22 am #

    Hello Sarah,
    Thank you for taking the time to answer my question. My concern came mostly from my practise of using the egg tempera medium where rabbit skin glue and traditional glue gesso has been the mainstay for centuries. However, I never thought of SID before. My panels are usually on hardboard and I know the old panels (in the days before hardboard was invented) were wood. Although I have never seen SID occur in my work, even on the older pieces, I am starting to be concerned.
    The term “open” and ” closed” gel medium are unfamiliar to me. What do they refer to?
    With thanks,
    David

    • Sarah Sands February 21, 2017 at 2:33 pm #

      Hi David –

      If you have not noticed or seen SID within your work then you are fine. It develops during the process of drying, so once an application like your traditional glue gesso has dried, then the danger of SID developing has passed. In other words, it will not all of a sudden appear years or months later. So all in all, I think you are fine. As for the use of the word ‘OPEN’ it refers to the name of a paint line we make, which in turn took its name from the fact that it has a very long’open’ time, meaning that the paint dries very slowly. None of our acrylics are described as ‘closed’, which in this case is not really the opposite. Rather we talk of them as being fast-drying. Or simply ‘regular’ acrylics.

      Hope that helps!

      Best –

      Sarah Sands

  4. David McKay February 24, 2017 at 8:22 am #

    Yes, that helps a lot. Thank you Sarah. David

  5. Sean February 25, 2017 at 12:38 pm #

    Just curious, but when you say that shellac based primers are suitable in some circumstances are you also saying that they’re archival?

    • Sarah Sands February 27, 2017 at 9:45 am #

      Hi Sean –

      “Archival” is one of those terms we try to avoid as it is so ill-defined; originally referring solely to the field of maintaining archives – as in documents – it has become something of a generalized catch-all phrase since then. Shellac has been used in the fine and decorative arts for centuries, both as a protective coating as well as a binder for things like India Ink, so we feel it is a suitable material as long as its vulnerabilities are taken into account – mainly that it will yellow and grow more brittle over time. However, used as a primer on an inflexible support, those issues will not really come into play so it should prove to be quite durable and longlasting.

      Hope that helps!

      Regards –

      Sarah

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