I’ve begin to notice that I work better on rendering subtle flesh tones when I’m working over slightly dry paint.
Working wet into wet can be very difficult especially when you are trying to accomplish very subtle transitions between values and colors. What happens is that you always seem to pull up the layer below, mixing with your current stroke and your subtle color shift is not so subtle anymore. You can lay wet color on top of wet color but in order to achieve this without disturbing the color beneath you have to fill the brush with a large amount of paint resulting in a thick impasto.
On the other end of the spectrum is working over dry paint. In this instance I find it hard to move the paint around well. I inevitably put a general value or color over the dry area then begin working into it. In some cases I’m back to the wet on wet problem. Now, this could be fixed by putting a very light coating of linseed oil over the dry area before working into it. Sometimes called “Oiling in” or “Oiling out”… I’m not sure if it is “in” or “out”. Anyway, that method tends to thin the oil paint out resulting in a transparent effect that is not that easy to control. Regardless this could be used as an advantage depending on what you’re trying to do.
What I’ve found recently is that working into slightly dry paint is the best. It is not too wet that it disturbs and picks up easily and it is not to dry to prevent blending into it. I can easily go from laying color over to blending into.
There is another complexity that is thrown in here also. These subtle working properties of picking up or laying down paint depends on the type of brush you are using. A hard bristle brush will scoop into previous paint and move it around unless you use an impossibly light touch. On the other hand using a synthetic sable lends itself better to working over slightly dry oil paint.
But, this all relative. If you want to work like Lucean Freud, then everything I said above can be thrown out the window. So there you go, the complexities of art.