Never mix oil colors with acrylics or paint acrylics on top of oils. However, oils may be applied over acrylics in a few circumstances. Acrylic gesso is a great primer for raw canvas, and many oil painters prefer to use this type of gesso before oil painting. Acrylic paint may also be used in very thin, “lean” layers to create an under-painting. Acrylic paint is more flexible than oil paint, so thick applications of acrylics under your oil color may lead to cracking.