Acrylic primer cleaning

​Speaking as an artist and not a conservation professional, I’m sure there are some reasons why a mixed acrylic gesso/oil paint surface might be more challenging to maintain and treat than a pure oil painting but I consider the instance of some exposed acrylic ground on an oil painting to be “within tolerances” and not a malpractice. Especially if that approach is helping you achieve a specific visual effect, I would not change course. That said, I think documenting the materials used and sequence of application goes a long way toward ensuring the long-term survival of your work, and it’s a great service to the collector as well. My advice would be to use a bona fide picture varnish appropriate for both oils and acrylics where the resin and solvent are known, and include this information with the painting. 

Acrylic dispersion paints and grounds are more open than well bound oil paints. This does mean that they are more likely to absorb and imbibe grime over time if left unvarnished (very lean oil paintings are also susceptible to this). This becomes a moot point if one follows Matthew’s advice. However, varnishing is an aesthetic choice should not be performed if it violates the artist’s central vision. If one can varnish, all is well; if one cannot, it is probably not a major issue. As to acrylic dispersion being more sensitive to some solvent.
Yes, that is true as it relates to some of the solvents used by conservators. Acetone, ethanol and toluene are particularly damaging. This just means that you should chose a varnish that does not require such polar solvent for removal and that you document your material and varnish choices on the painting. You can find threads and resources about this on MITRA. I would probably choose a varnish composed of regalrez in a low aromatic solvent. This will remain eternally soluble in odorless mineral spirits. That solvent has been shown to have little effect on acrylic dispersion paint films.

​I agree that if you opt to varnish, then Regalrez should be an excellent choice. We have tested removing Regalrez from a few acrylic surfaces and found that on most colors it can be removed without problem but some colors can be prone to color lift, such as organic synthetic pigments. However, removing Regalrez varnish from an acrylic gesso should not pose a problem. If you don’t want to varnish and leave acrylic gesso/paint exposed in some areas, then conservators would find ways to treat such a painting as well. It is common practice to develop individual cleaning plans for specific areas of a painting. What you could do, while the paining is in your care, is to dust it once or twice a year with a clean soft brush to reduce dirt collecting on the surface and getting imbibed in the paint layers.  
Mirjam Hintz (Golden Artist Colors)

​Hello Marc, please note the correction in my reply- I meant to say that organic pigments tend to be more sensitive to color lift. In this pigment ID chart one can sort colos by pigment identification:  https://www.goldenpaints.com/technicaldata/pigment