"When I was about to start an oil painting on canvas, I noticed I didn't have a particular green I wanted in oils, but I did have it in acrylic. As the canvas was suitable for both acrylics and oils, I decided to sketch the outlines of the elements with acrylic and blocked in some areas using acrylic green. Then I finished the painting with my oil colors. Is it okay to use oil paints on top of acrylic paints, or should I expect any problem on this paint in the future?" -- Alejandro.
Provided the canvas has been primed to be suitable for both oil paints and acrylics, it's fine to start a painting with acrylics and then finish it in oils. A lot of artists use acrylics to start a painting because they dry so much faster, then finish the painting in oils. Just be sure that the acrylics have dried completely (all the way through, not just touch dry on the surface) before you start with the oil paint. (If in doubt, leave thin acrylic paint at least 24 hours, or several days if it's thicker, even in places.) What you can't do is to start a painting in oils and then paint acrylic on top of this.
Don't use the acrylic paint too thickly and smooth as you don't want to create a smooth surface oils can't stick on. (Thin glazes of acrylic on canvas will likely not fill the tooth of the canvas completely.) The bond between oil paint and acrylic is a mechanical one, not a chemical one (think "glued" or "stuck together" rather than "intertwined" or "mixed").
If you're worried about the issue of the different flexibility of acrylics and oils once they've dried, paint on a rigid support (such as hardboard) rather than a flexible one (such as canvas).
Mark Gottsegen, author of The Painter's Handbook, says there has been "anecdotal reference to the failure of oil paints applied over acrylic... but no hard and consistent evidence from conservators. A lot of the failures of paintings in general can be traced to faulty artist techniques..."1
An information leaflet published by Golden Artist's Colors on priming says: "While we have done studies of the glossiest of our acrylics under oil paint films and have not seen any signs of delamination, we want to err on the safe side and suggest the films should at least be matte finishes."2
References:
1. Mark Gottsegen, Acrylic Underpainting for Oils, AMIEN (Art Materials Information and Education Network). Accessed 25 August 2007.
2. Priming: Acrylic Gesso Under Oil Paint, Golden Artist Colors. Accessed 25 August 2007.
