MITRA Forum Question Details

Hi - I would be happy to share what we know from our testing and research, which should be applicable to a wide range of professional brands of acrylics. See my notes below:

1. As I understand there are three mechanisms that enable adhesion of a new layer in a painting:   a) chemical bonding, b) absorption (wetting of the existing surface by the applied layer), and c) mechanical bonding (interlocking with rough/textured areas). Is this accurate? 

Adhesion is a complex, broad field, so a true listing of all the mechanisms would be a touch longer, but yes, those are the main ones to be concerned about with paint.

2. When applying the first layer of oil paint over an acrylic primer I assume that there will be no chemical bonding? 

That is correct. I also think we have a tendency to overestimate the role of chemical bonding in the adhesion of oils on top of oils. At least in terms of durability. Oil is just not a particularly strong adhesive, and while it will form a soft gel on its own, it will not possess strong cohesive forces and the bonds can be easily torn apart. It's not to state that chemical bonding doesn't happen with oils, but I continue to believe that absorption and mechanical adhesion play larger roles than we realize.

3. Acrylic grounds have calcium carbonate added to promote mechanical bonding. Is this intended to be the primary adhesion mechanism between the first layer of oil paint and the acrylic primer?

Yes - but, at the same time, for mechanical adhesion to be effective you also need absorption/wetting, and with that comes interactions between the molecules via van der Waals forces. So various mechanisms are still playing a part. I would also want to point out that calcium carbonate plays the same role in a traditional oil ground, which ideally should always be slightly absorbent and toothy as well. Ultimately, I think mechanical adhesion and absorption are playing outsized roles in both systems. After all, how comfortable would any of us feel if we had to start an oil painting onto a very glossy and smooth oil ground. A rougher, matte surface, in the end, provides maximum adhesion in any system - acrylic to acrylic, oil to oil, oil to chalk gesso, oil to acrylic, etc - if only for the increase in surface area and more opportunities for mechanical interlocking.

4. I've noticed that some brands of acrylic primer tend to be less absorbent than others - as evidenced by less sinking in of the first layer of paint. Also, "non-absorbent" acrylic primer has appeared on the market. It begs the question, what role does absorption play in the bonding of oil paint to acrylic primer?  Can too much absorption rob the oil paint of binder? Can too little absorption limit adhesion? 

The degree of absorbency will definitely differ between brands based largely of the amount and type of solids being used, as well as the percentage of binder. We are not familiar with any 'non-absorbent' brands and it might be that the phrase is being used more for marketing than anything else. I can share that when a large range of manufacturers helped create the ASTM Standard for Acrylic Dispersion Grounds (aka acrylic gesso) we agreed that a ground had to absorb the oil to some degree and that oil that simply sat on the surface, with little to no penetration, would constitute a failure as much as the other extreme, where the oil quickly traveled through to the canvas. If you are experiencing things being too absorbent you might try adding small amounts of a matte medium to adjust the degree to your liking. Lastly, the number of layers you apply can make a difference. For example see the following article where we show some of this - keeping in mind that the same findings are likely true for any number of companies, but of course always test any brand to make sure you are getting the results you want.

http://www.justpaint.org/oiling-out-of-dead-colors-in-oil-paintings/

To answer your other questions, too much absorption can definitely leave a film underbound, chalky, and dead, So not a good thing. As for the opposite, while we have seen repeatedly in our testing that oils can achieve good adhesion even to glossy acrylic films, since at the microscopic level even a smooth acrylic paint layer has innumerable 'worm holes' that oil will penetrate down into, we would never consider this ideal and the adhesion, while possibly adequate depending on your needs, would certainly not be maximized.

Hope the above is helpful.

We are always happy to share our thoughts and, yes, having this posted on a public forum definitely has advantages as it allows for others to access it as well. Best of luck going forward.