MITRA Forum Question Details

We are sending this question to a few of our moderators who are more directly involved in acrylic dispersion media but I will attempt to provide a preliminary response. First, I do need to ask what you mean by too much extender. Are you adding additional clear acrylic dispersion medium to the paint? If that is the case, you are not really diminishing the binder and the paint film should remain resilient. If you mean adding a dry powder like chalk or additional pigments to the paint, you are in essence raising the pigment load and weakening the binding strength.

First, to my mind, I think that we need to separate the issues of acrylic dispersion paints that are thinned with too much water with that of adding too much retarder. The first is a physical alteration while the second is also chemical. I cannot really comment on the latter and we will have to wait for someone with greater expertise in this area.

I do think that applying a thin dilute layer of acrylic medium over the underbound paint could help to stabilize the layer. Generally, applying a fatter paint layer over an overly lean layer does not always solve the problem, this is certainly true of oil paint, but even in that circumstance there are times when it is helpful. To be clear, though, one should never apply straight oil over a layer of oil paint unless this area will receive subsequent layers of pigmented paint (see our “Varnishes” document in our RESOURCES section for more information about “Oiling out”). The use of clear surface applications of medium is far safer in acrylic dispersion paints where the yellowing of the binder is relatively insignificant.

The answer to “b” is likely similar but I will leave this to those more experienced in acrylic dispersion paints. Probably the medium heavy application of additional acrylic dispersion paints would work. The efficacy of the superimposition of oil paint would depend upon the degree of stability of the lower acrylic dispersion paint layers. Likely, a lean acrylic underlayer would function similarly to a lean acrylic ground and the results would be satisfactory. An extremely underbound layer, however, may be so thirsty for medium, or so friable, that either the top layer of paint is drained of its binder or the adhesion to the lower layers is compromised. Again, others may be able to provide a more precise answer.

Hi -

Similar to Brian's reply we would want to understand what you mean by "over extended" in terms of water. This is especially true as this is an area that is likely in our top 5 misunderstandings, at least in regard to our acrylics and likely other professional brands. In terms of Golden acrylics, our standard Heavy Body and Fluid acrylics can easily be thinned by up to 1:1 with water while still producing a very durable film. And in fact in many tests on Plexiglass - a very nonabsorbent surface - we still had good adhesion at ratios of 1:2 and 1:3 paint to water ratios. So acrylics can take a lot of thinning and still be fine. And then too a lot depends on the conditions of application. For example, dilute a tablespoon of acrylic paint in a gallon of water, then let evaporate, and you will find that you still end up with a fine acrylic film at the bottom of the bucket! The reason being that the initial dilution is still quite contained in area.

As for your concerns, if worried you went too far in thinning, one quick test would be to rub an area lightly with a wetted Q-tip and see if there is color lift or an obvious powdery surface. If yes, or if just wanting to play it safe, you can carefully apply a layer of an acrylic medium over the surface and that will fully consolidate and lock in the area and allow you to freely apply more acrylics or oil paints on top. In the case of oils, however, we would recommend something like our Fluid Matte Medium as a form of 'clear gesso' that will provide adequate tooth and adhesion for oils.

And if misunderstanding your concerns somehow, or you have further questions, just let us know.

Hope this helps!

Sarah Sands

Senior Technical Specialist

Golden Artist Colors 

​First, it's important to understand the difference between "retarder" and "extender". Acrylic mediums that are sold as "extenders" contain the same polymer dispersion base as acrylic colors, so adding a lot of medium doesn't result in a weaker film. Alternately, most retarders for acrylic paint are not acrylic mediums; they are mixtures of propylene glycol, water and gelling agents. Glycols act as coalescents in acrylics, moderating the rate as which the film forms and achieves full integrity. Adding extra glycol and water-gel keeps paint wet longer, which is advantageous when painting outdoors on a hot, dry day, or when more suave blending is desired. Since retarders don't usually contain any acrylic dispersion base, they don't reinforce film strength the way acrylic mediums do, so use of the minimum effective amount is recommended. Excessive use of retarder (with our product, more than 25% by volume) can result in a porous film that stays sticky for a long time, and which permanently becomes tacky in warm display/storage conditions. Professional-grade artists' acrylics can be thinned with quite a lot of water and still achieve a durable film, but I have seen really watery, thin applications that don't adhere well. Thinning with a lot of plain water also temporarily lightens mixtures, making it hard to judge the final appearance from palette to canvas. Low-viscosity mediums (e.g. Acrylic Sizing and some of the GAC mediums) added to thinned paint can boost adhesive power, plus help reduce the difference in appearance between wet and dry colors.

Matthew Kinsey Utrecht Art Supplies

Just wanted to concur with Matthew Kinsey, from Utrecht, about the distinction between a retarder and an extender/medium, ​an issue I really did not address earlier. As Matthew pointed out, putting an excessive amount of Retarder into a paint film - with our products, we recommend no more that 15% Retarder - can result in a persistently sticky, water-sensitive film. Assuming that you do not have that problem, and the film feels fully dry to the touch, we would still recommend testing for water-sensitivity just to be safe. Especially if you suspect that you went well beyond the maximum recommendations of the various brands. To do this, as mentioned before, just use a wetted cotton swab and rub slightly, looking for color lift. If nothing comes up, you are likely fine. If you get staining, then carefully applying a light coating of an acrylic medium of some sort should seal it in. If you get excessive color lift, or have a powdery surface, then you might need to first consolidate and lock down the surface with a light spray of something similar to our Archival Varnish aerosol, and then follow with an acrylic once allowed to dry for 24hr.

Hope that helps  -

Sarah Sands
Senior Technical Specialist
Golden Artist Colors

​Based on Matthew and Sarah's comments above as well as my own understanding of retarders I would be inclined not to lay down an initial layer of retarder and/or add too much retarder in general. I think they have already outlined some of these issues but I would reiterate that even if a layer of retarder is below an acrylic layer with an appropriate amount of retarder, the lower layer could very well effect the integrity of ANY of the films above it....if the retarder has nothing to "retard" then any subsequent layers on top may have a hard life in the future. But perhaps this is what you are going for? You can certainly experiment but I would consider what folks have stated here....if you do decide to go that route it would be great to record your technique/materials on the back of the painting, not just for future preservation but for yourself as well, to help keep a record in case future problems should arise (or even if everything works out). 

​Are you avoiding using oils for any particular reason? Perhaps for health concerns?

​Well if not for number 3 we might be able to offer some tips regarding the other issues. It is of course possible to paint without solvent....it just takes some getting used to. As far as driers go, unless you are using solvents and getting paint (with solvent) on your hands, there is very, VERY little chance that such materials would transfer through the skin. Same with oil paints containing lead, cadmium, cobalt, etc. Of course you can always wear gloves. But it sounds like acrylic is something that helps you to ease your worries so there is no problem with that. Many of us have already stated above our concerns about painting over a layer that is rich in retarded so as long as you are aware of the risks I would just encourage you to record your materials and techique on the back of your painting for future reference.