MITRA Forum Question Details

Just as waiting 6-12 months for an oil ground to cure is better than a week or two, waiting longer after applying acrylic gesso can only help that film fully dry and acquire more of its final, long-term traits. That said, the vast majority of the moisture that will come out of an acrylic gesso film - especially on canvas - will do so in the first 72 hours, which is what we give as a minimum timeframe, while two-weeks is our usual recommendation - but we would not quarrel that, if you can wait, a month is even better. The fact is, there will always be moisture in the gesso - just as there will always be moisture in the canvas and wood we use, as materials reach an equilibrium with their environment. For an acrylic film, in typical ambient conditions, that is in the 11-16% range, which is about the same level of moisture in wood, which of course can be primed with oil-based paints. You can read some of our research in this area in the following issue of Just Paint:

http://www.justpaint.org/wp-content/uploads/2015/11/jp27.pdf

As you look at the graphs there keep in mind the “thinnest” film shown is quite substantial - 1/16” - and a brushed on film of gesso, especially on canvas, will reach those equilibrium levels much much faster. And keep in mind, that on canvas evaporation will continue to happen in the rear of the canvas, so even after one paints further consolidation is taking place.

Hope that helps. On a final note, in my experience, if you prime several canvases and decide to paint on one of then after 3 days or a week, the others will almost invariably sit around and continue drying and by the time they get used a month or more would have passed.

You are very welcome. "Cured" in quotes - just as you did -  is definitely accurate as technically curing refers to chemical changes usually triggered by a substance or energy source. Thus you can have moisture, heat, and UV cured coatings, as well as two-part systems where one part acts as the catalyst. In water-based acrylics, however, the polymer chains are already fully formed and suspended in a dispersion and a more accurate description is that once the water evaporates and the polymers are pushed together, they begin a process called coalescing where the polymers entangle and fuse together into a continuous film. It is that process that continues for quite some time - certainly out to a month or more - but the question becomes at what point can it safely be painted over, and at least for us at Golden, 2 weeks is a good practical timeframe to aim for, with a month being more of an ideal.

Sarah Sands, Senior Technical Specialist, Golden Artist Colors

This is always one of those difficult questions to answer. As you gather from your own experience the risks are very minimal and usually nothing happens, and because of that, I don't want to raise incredible concerns. The odds of a problem occurring are probably very low. But.... (stalling as I put on my technical hat).......can I imagine scenarios where something occurs? Sure. Higher humidity could slow the evaporation of moisture enough that the oil paint does not penetrate as deeply, and so develops less mechanical adhesion. But I think it would need to be extreme to really be a major issue. Also, keep in mind that acrylics are alkaline in their wet state - and contain a small amount of ammonia - so until the majority of the moisture has evaporated the canvas surface could still be alkaline as well. Alkalinity, in turn, can cause the fatty acids in the oil to saponify, which could also impact adhesion. Because it is just good advice to keep ammonia and oil paint away from each other, we generally prefer giving the surface extra time to fully dry and be truly pH neutral.

That said, we have never seen the above issues come up in our testing, nor in all the years we have fielded calls and inspected canvases. So my guess is that it is extremely rare and requires something of a perfect storm. But......tech hat again......perfect storms DO happen, and if it happens on a really important piece, that can be heartbreaking.

In the end, the 24-48 hrs. you are waiting is shorter than the 3 days we recommend as a minimum, but we are also erring on the side of caution and trying to give advice for people living in a lot of different environments. Also a brushed on acrylic layer is just so thin that in truth the majority of the moisture will easily be gone in that initial timeframe. At the very minimum, however, I would say do not paint on any water-based coating if the surface still feels cool or clammy, as that would clearly indicate a good deal of evaporation is still taking place. And as I always like to note, if you can prepare several canvases at once, then even if you use one after a day or two, the others almost invariably will sit around for a week or two.  

Hope the above is helpful. In the end it is all a matter of risk management. If you always gravitate and want to adhere to the absolute best practice, and have minimal to no risk, wait two weeks. If a small amount of risk is tolerable, wait 3 days. If you feel okay pushing that envelope slightly, then your 24-48hrs is likely fine. It reminds me a bit of the quandary whenever reading medical statistics. If doing something increases the likelihood of an illness by 400% that sounds shocking, and one quickly promises to amend their evil ways, but if that translates to actual incidences going from 1% to 4% that might seem okay for the tradeoff, and french fries it is!

​Hi Ron -

You'll be fine.....always a pleasure to chime in.