(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987 revised 1998)
Although water‑based paints, such as acrylic emulsion paints and vinyls, should never be painted over any oil‑based paint, oil‑based paints can safely be painted over water‑based polymer emulsion paints. There are always exceptions, but this is rule that manufacturers of both types of paint offer. Many artists take advantage of this ability of oil paint to adhere to acrylic paint films, because it is an economical means of painting to start with acrylics and finish with oils. And, the acrylic underpainting dries quicker allowing faster painting.
The use of wax in encaustic painting is a long‑established, safe painting method. However, there have been some problems in the use of encaustic paints and oil paints within the same painting. Paintings in which oil paint has been applied over areas consisting of high proportions of wax have cracked and sometimes the paint has fallen off the surface. Wax appears to dry quickly, but, in reality, it only sets up quickly, for it takes years to cure, certainly far longer than the oil paint applied over it, and therefore violating the rule of never applying a faster‑drying paint over a slower‑drying paint. The use of encaustic should not be a start‑and‑stop operation. Once wax is used, it is safer to finish with wax and not with oil. In the chapter on media we discuss adding wax as a medium to oil paints.
The mixing of drawing materials such as pastels, painting sticks, and crayons with oil painting though not popular is done. Materials such as these should be reserved for the final stages of a painting, the last material to be applied. Oil pastels, crayons, and painting sticks are made with or wax and pigment and should be treated as an encaustic paint. Soft pastels are more difficult to use safely because of their chalky nature, which can give unpredictable results. However, charcoal has traditionally been used for preliminary, light sketching on the canvas before the painting is started, and can be used safely in moderation if the charcoal is sprayed with either a workable fixative or retouch varnish before the application of oil paint.
Although the use of mixed media can be a powerful method of expression, many an art conservator has had to repair works in which materials were thrown together carelessly.