.Revisiting Oils Over Acrylics
April 26, 2022 by Greg Watson
For some time, our recommendation for artists using oils over acrylic has been to work over harder, matte acrylic surfaces and avoid working on softer gels and gloss products. Our intention was to optimize the level of adhesion that would be achieved on a toothier surface as well as avoid the potential for future cracking as the oil paints become more brittle. While we have not seen adhesion problems of oils on any type of acrylic, recent testing has shown the potential for cracking in certain instances and conditions when applying artist oils over all of the many brands of glossy acrylics we have tested. Though we have not received notice from artists of this phenomenon, we are able to repeat this specific type of cracking in our Lab.
What follows is an overview of our testing, results and updated recommendations for applying oil colors over acrylics.
Methods & Materials Tested
Applications included multiple substrates: stretched canvas, unstretched canvas, canvas board, wood panel, polyester sheeting and lacquered paint testing cards. The type of substrate has minimal to no impact on the outcome. Lacquered paint testing cards were the preferred substrate to create repeatable and measurable results.
Multiple acrylic grounds, gels and mediums from different brands were evaluated, including acrylic gessoes, molding pastes, matte mediums, acrylic gels and mediums with different sheens, as well as acrylic colors.
Multiple artist oil brands and colors were evaluated. Several colors were selected for increased study because they exhibited the strongest effects of cracking.
Acrylic mediums and grounds were applied onto lacquered cards using a draw down blade to assure uniform thickness as well as to understand how various thicknesses of these mediums might exacerbate the resulting effects. Several substrates with older acrylic applications were also evaluated. Materials were allowed to dry for at least three days in ambient conditions before oil applications. The age of the acrylic had insignificant impact on results.
Oil paints were knifed onto the test surfaces and thicknesses were measured using a micrometer. Tests were performed with and without the addition of alkali refined linseed oil, alkyd medium and stand oil.
Results: When is Cracking Happening?
Our testing indicates that if cracking develops in the oil layer because of this phenomenon, it is during the early stages of drying – as the paint transitions from being wet to touch dry. Fast-drying colors like Mars Yellow or colors that are susceptible to this type of cracking like Ultramarine Blue, may develop hairline cracks within a day or two. Slower drying colors can take weeks or even a month to show any issues, if ever. It appears this cracking does not develop further once the color is fully touch dry.
Summary of Findings
We have not found cracking in oil applications over GOLDEN Acrylic Gesso, Sandable Hard Gesso, Molding Paste, or surfaces sized with Rabbit Skin Glue when these products are used on their own with oil paints or grounds applied directly. This is also true for most other brands of acrylic gesso we have tested.
We have not experienced adhesion or embrittlement issues related to painting oils over acrylics (other than zinc rich oil paints).
Cracking is mostly happening in oil paint or oil ground with no oil or solvent added, applied directly out of the tube or container over the acrylic surfaces. These oil applications are roughly between .05mm and .25mm, which is the thickness of 1-5 sheets of office paper. Layers that are thinner or thicker do not seem to crack.
Glossy acrylic mediums and gels are associated with the most cracking in lean oil layers applied on top.
Satin, semi-gloss and matte acrylic mediums, acrylic colors, and aggregate pastes and gels can be associated with minimal or microscopic cracking in oil layers.
Here are some recommendations for artists using oils over acrylics with suggestions for mitigating the potential for cracking. Because our research is evolving, these recommendations may change. An editor’s note will appear on the top of this article if we change these recommendations in the future.
Use three or more coats of professional, white acrylic gesso as a ground for oil painting.
Avoid glossy acrylic products in any preparatory layer of an oil painting. This includes products that we have recommended for sizing or preparing surfaces in the past, such as GAC 100, GAC 200 or GOLDEN Gloss Medium.
Add oil painting medium or binder to oil paints if working over satin, matte, aggregate products or acrylic colors. We have found adding 5% or more of medium to the paints significantly reduces the potential for cracking.
Acrylic Isolation Layer Before Oil Paints Gesso helps block cracking associated with underlying gloss acrylics with 3-4 coats, however, some artists want to be able to see through to the underpainting or the substrate. We have previously recommended that a layer or two of Fluid Matte Medium or Matte Medium be applied over glossy acrylic sizing or acrylic colors to provide additional tooth before subsequent application of oil paints, but in some cases, their addition makes the cracking even worse. For this reason, we no longer recommend Matte Medium or Fluid Matte Medium as an additional isolation layer over other acrylic products in preparation for oil painting. Instead, modify oil paints with 5% or more medium or oil binder if painting directly over acrylic colors or any matte acrylic being used as a translucent ground. Acrylic Color Underpainting We have not completed a comprehensive review of all of our acrylic colors and formulations, but so far have seen minimal cracking over acrylic colors from multiple brands, as compared to acrylic mediums. Generally, if acrylic colors are being used for underpainting oils, we are currently recommending modifying oil colors as described below. Modify Oil Paint Applications with Oil or Medium The acrylic products listed in the chart above are not associated with cracking and can be used without concern. However, when other acrylics are used in the under layers, including acrylic colors, we recommend modifying your oil colors before painting. Adding 5% or more of stand oil or alkyd medium, or 10% linseed oil to the oil paints can reduce or eliminate cracking entirely. While adding just a little solvent to color or oil ground did not change outcomes, we found that very thin solvent-based washes of color did not seem to crack in most applications. In Closing This research exposes limitations to how oils and acrylics can be used together. While this information might raise concerns, it is good to know that cracking is avoidable with certain working methods. We are also trying to understand why this phenomenon seems to be unreported or under-reported in the normal course of painting oils over acrylics, a method that has been used successfully for decades by artists. We are committed to continuing our own research and are in communication with multiple conservation research partners to better understand this issue so we can provide artists and other art material manufacturers with improved recommendations. Please contact us at firstname.lastname@example.org or call 800-959-6543 with questions