MITRA Forum Question Details

I will send this to our moderators who are experts in acrylic dispersion paints and grounds. I do want to first make clear, zinc white has not been shown to be problematic in acrylic dispersion mediums in any manner. It is the interaction with zinc oxide and fatty acids contained in oil paints and likely egg tempera, etc. that are problematic. Acrylic dispersions are vastly less likely to be a problem.

Perhaps, however, you meant that you overpainted with zinc white containing oil paints, which is a bit more problematic. Clarification would be helpful.

​A heavy-bodied acrylic dispersion painting ground (gesso) would work for this application. For a denser, heavier product, Acrylic Modeling Paste or similar would be good. While modeling paste isn't made expressly for use as a painting ground, most brands do provide enough absorbency and tooth to accept at least moderately thick oil paint. The technique you describe is probably best used with a rigid (panel) support, especially if some force is applied, which might distort stretched fabric. I think palette knives and silicone-tipped painting "wands" would be a good choice for working into the primer. Sgraffito with the brush end will also work, provided the support material isn't exposed. (Preparation in advance with acrylic sizing or a completely dry priming layer could help prevent exposing the panel.) Embossment of the partly-dry ground should be OK, but I wouldn't consider any phase of partly-dry acrylic ground to be "open"- if the film is disturbed or broken when skinned over, it might still be OK, but it's really best to manipulate acrylics when wet. 

​The maximum thickness of gesso that can be applied to canvas is determined by the flexibility of the product. Some products are more prone to cracking and don't perform as well in heavy coats, so it makes sense to be familiar with the properties of the brand you use. Modeling paste can be particularly brittle. Preserving the canvas weave is partly an aesthetic choice, but it also provides more surface area, which helps paint adhesion. Smooth primings made with a good quality gesso are fine, however, especially if heavy impasto won't be used. Again, I was recommending manipulating the wet primer or embossing the dry film, not specifically carving or cutting into it. I'm sure scratching and cutting into the primer is OK, but it seems to me that there is a chance of  reducing the integrity of the ground by doing so. 

Hello,

OPEN Titanium White or blends of OPEN Gel or Titanium White with Molding Paste should give you increased open time. Only bear in mind that OPEN paint layers of more than 1/16 inch / 1.5 mm (wet film thickness) can result in excessively long drying periods and persistently soft or tacky layers. Therefore, it would be better to only add as much OPEN as necessary to Molding Paste or thick Gesso. Before continuing to paint with oil, the acrylic layers should have dried thoroughly. For a thin layer of fast drying acrylics we would recommend 3 days, with OPEN this can take several weeks.

One more aspect to consider when painting with acrylics on panel is Support Induced Discoloration (SID), especially when using slow drying acrylics and when wanting a truly white surface. To block the discoloration from SID you can apply 2 coats of Golden Gloss Medium, which has proven the best SID blocker in our testing. But a commercial stain blocker from a hardware store should work as well. This will not only block SID but also seal the surface absorbency of the panel and hence give you a little more working time for inscribing.

The good news is, that we have had only good experiences with oils over acrylics. Sarah Sands has done a lot of testing on 15 year old samples (oil over acrylic), which she bent and flexed and the oil layers did not cleave. Not even those samples where oil paint was applied over smooth glossy acrylic glazes.

Let us know if you have more questions.