Thank you for this excellent question Jose. As with most aspects of an activity and complex as painting–there is no fast and easy, global answer. Whether or not “oiling out” is the right move in a painting process depends on quite a number of variables.
First I should say that I find far too many artists mindlessly include steps or stages in their process because someone else did or they have been told to. They have no real grasp of how certain factor, action, or particular material is affecting the overall dynamics of all variables involved. I urge all of my students to understand WHY they are including some variable before it is deployed. As such I am very glad to see questions like this.
As to sinking/oiling: One very peculiar behavior of oil paint that most painters are familiar with is the change in appearance that seems to take place when a paint film is curing. The paint may appear to lighten significantly, resulting in a chalky surface–or become blotchy with some areas appearing more matte than others. This is often far more evident in darker colors. This perceived change is due to the oil in the paint being drawn in to the absorbent surface that you are painting on. This effect is almost always eradicated when the painting is varnished so do not be alarmed if a drying painting begins to appear this way. However, when sinking occurs on a painting that is still in progress, certain steps may need to be taken to ensure it does not interfere with your process.
So how might sinking interfere with an ongoing painting process? Well, one way emerges from the perceived change in the paint value/color relationships. Let’s say that you have established a strong collection of value/color relationships that you feel communicate your subject successfully. Let’s also assume that the level of refinement that you seek will require the application of additional paint layers. The issue that will arise is that your judgements from here on out will most likely be based on the current (sunken) appearance of the work. The change in appearance due to sinking does not reflect how the painting will look when it is varnished—which may result in significant errors in judgement and adverse results.
For example, if we look at this Gradation Pattern exercise from the Language of Painting we can see how the painting looks wet versus with sinking during oxidizing (drying). If we match colors/values based on the “sunken” appearance, a later applied varnish (or other restoring agent) will indeed yield a significant disparity.
We contend with this issue as by “oiling out” only those areas necessary. Our process for this is often far more conservative than most. Only the area to receive paint and immediate adjacent regions are oiled out so as to maximize effective color and value judgement. Nothing more—
Here is an excerpt from the Language of Painting that touches on Sinking, Oiling out, and Sanding:
Now Jose, I agree with you 100000%. I HATE trying to paint into a slippery layer of oil—so again, I add JUST ENOUGH to activate a color/value. Again—nothing more.
Some however do like to maximize the fluidity or flow of paint so they add the “couche” (that you mention) before working on a dry painting. For those who haven’t come across this term before, “couche” is a French term for “layer,” and in the context of painting–a couche is a thin layer of oil that you spread over the entire work or a region that you are about to work on. The oil may significantly increase the “flow” of the paint. A couche will also add the benefit of restoring the initial appearance of sunken colors—but as the description suggests–it tends to be a far more cavalier or liberal application of oil. For me, the process of oiling out is an extremely conservative, localized couche.
I hope that this explanation is helpful and will allow you to make the right decision for yourself.
Additionally, Natural Pigments founder George O’Hanlon has made available some interesting material options that may lead to the avoiding of “sinking” altogether. If you would like more information regarding George’s materials I recommend research into some of their products here: http://www.naturalpigments.com/
Happy Painting!