If you follow the practices of the old masters, you will prepare the canvas with a rabbit skin glue size and a traditional chalk gesso. The size, which is a barrier coating, and the gesso layers protect the canvas. Without these layers, the oil in the paint will gradually rot the fibers of the canvas.
Because the chalk gesso is very absorbent, the oil in the first layers of paint will soak into it like a sponge. When the oil sinks into the gesso, the paint loses much of its gloss. This can also happen when painting one layer of paint over another. In these cases, the oil from the top layer can also “sink in” to the lower layer leaving a splotchy appearance with spots of the painting with a matte finish. What contributes to an uneven appearance is that the oil of different colors can be absorbed into the painting at different rates.
To even out the finish and bring out the color saturation, many artists will oil out the canvas during the painting process, brushing an oil medium mixed with mineral spirits onto its dull painted surface. Typically, artists will mix the medium and solvent at a 1 to 1 ratio. After completing this process, the artist can continue painting. Some feel that painting on the oiled surface facilitates blending of colors.
NOTE: If you oil out a painting as you are working on it, make sure that the painting is dry to the touch before application.
“Oiling Out” Techniques
Artists will oil out their canvases using different techniques. Some apply an oil medium/solvent mixture with a brush. Others use a rag. Using a brush is generally the preferred method. After brushing the mixture evenly on the surface of the painting, wait a few minutes before rubbing off the excess with a lint-free rag. This leaves just a very thin layer oil that you can paint on. Failure to mop up the excess can result in failure of intercoat adhesion, in which one layer of paint can flake off from another layer.
Other artists will apply the oil medium with a clean rag, rubbing it into the surface of the painting with a circular motion. If you use this method, continue to apply the oil until a uniform sheen is attained.
A splotchy appearance is certainly undesirable after you have completed your painting. Before you varnish a painting, you can oil out your work with a painting medium.
NOTE: you should only varnish a painting after it is completely dry. This can take as long as a six months to a year. Do not varnish a painting as an alternative to oiling out. Varnishing between layers of paint can compromise intercoat adhesion, because varnish is soluble.
While oiling out is most commonly used during the painting process and after painting is complete, some artists also use the practice to bring an old oil painting back to life.