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One possible problem with oil over acrylic is a variation of the fat over lean issue. Acrylic gives a very flexible paint film as compared to oil regardless of how much oil is used as a medium. Flexilble over less flexible is what is actually meant by fat over lean so acrylic paint would have to be thought of as fat even though there is no oil present. Putting acrylic over oil would have its own set of problems since acrylic dries so much faster than oil. In my opinion it just best to keep the two apart.
Acrylic grounds in the form of gesso is a slightly different thing. They are not as concentrated a paint, more absorbant, more open with an abrasive to grab hold of later paint layers. A well aged oil ground would be my preference, but I have no real concerns about using an acrylic primer.
Back to the original problem of green nasty thingies appearing on the canvas. Could it be mold? If the water hadn't fully evaporated before oil layers were put on it could result in mold forming due to the trapped moisture.
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