Q: When I paint on paper with oil paints, it is all right to prime it with acrylic medium?

A: Just as it is imperative that you prime canvas to protect it from acids in the oil, the same is also true with any paper support. Gesso, acrylic polymer primer, would probably work the best. Acrylic gesso can contain calcium carbonate (whiting) and titanium white. Therefore, it will not only protect the paper from acrylic medium but will also provide a satisfactory ground for subsequent paint layer to adhere to.