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(ssands)
Hi -
As Kristen mentioned, this is an area I have partly covered before and would also point you to the same thread. That said, I am also happy to touch on your questions here as well. In terms of the issue of oils over acrylics in general, we feel this article can serve as a good introduction and the points it covers should be applicable to any high quality professional brand of acrylics:
Using Oils with Acrylics
As for your specific concerns:
Adhesion: As the above article points out, we feel that oils will adhere to acrylics and all our testing has supported this. However let me throw in a few caveats. One, we know oil paints containing zinc have been linked to cases of delamination, including from acrylic grounds, and would recommend not using any oils that contain zinc - especially in an underlying layer. Two, while we believe and have tests showing that oils adhere to even glossy acrylic films (for example, oil grounds adhere well to our GAC 100 used as a size) we also know that in ANY system, including oils to oils and acrylics to acrylics, matte and toothy surfaces provide maximum adhesion. That is simply a function of increased surface areas and opportunities for mechanical adhesion through a 'lock and key' mechanism. Because of that we err on the side of caution and recommend keeping the acrylic underpaintings as matt as possible. If you have a variated surface, with differing degrees of absorbency and sheen, then applying a layer of a matte medium can serve as a translucent ground for the oil paint and provide a more uniform layer to work on.
Drytime between acrylic and oil: We know that in even a thin film of standard acrylics will lose moisture and other evaporatives, and start to approach an equilibrium with the environment, over the course of a couple of weeks. So we tend to recommend waiting two weeks as best practice, assuming the layers are the typical normal brushstroke thickness. Anything that is applied very thickly or uses very slow drying acrylics, such as Golden's OPEN, would need longer and could easily take a month or more. That is 'best practice'. At the same time, it is also true that the vast majority of moisture is lost from a thin film in the first 72 hrs and we feel that if you waited at least 3 days that any increase in risk is very minimal. But its not ideal. Besides, patience is a virtue!
Best thinning medium: Don't be afraid of water is the first thing to mention. At least in our own testing of our paints - and we assume this would extend to other professional quality acrylics - we have confidence that one has a very durable film even after thinning up to 1:1 with water. So if needing to adjust viscosity and get more flow, water will get you there faster than any alternatives. In terms of mediums, we would recommend going with matte mediums to provide that additional tooth that is helpful for subsequent adhesion of the oils.
Ideal substrate: An inflexible panel is by far the best option. Aluminum composite panels would be high on my list, as well as some high quality plywoods and, for smaller works, hardboard. This is really more about the oils than anything else, and would be true even if painting only in oils, even without multimedia concerns.
Watercolors for underpaintings: I don't know of a lot of testing around this specifically but I do know it is mentioned as a technique in some 19th century works in Leslie Carlyle's The Artists' Assistant. Overall I think if it is kept thin and on an absorbent surface it should be fine. But others here might have additional thoughts.
Hope this helps!
