After I'm done with the acrylic stage (and it is completely dry), I seal the entire canvas with Winsor-Newton's Liquin.
My first layer of oil on the background. I want to create an outdoor scene. As you can see it is messy and moody and neutral.
With a paper towel in one hand and a brush in the other, I begin to "push" the wet paint around.
I am only using transparent colors at this stage. If I begin to use opaque colors, it will turn into mud...ick.
I generally have something in mind but this is a good time to "play around" with color and value.
Background is done and I'll let it dry. Then I use soft chalk or charcoal to draw guidelines on the jacket. I keep it very light and it just melts into the paint when I cover it over.
To paint the color white, I use Raw Umber + Titanium white and this mixture is slightly cooler than what I can get with the acrylic colors. However I match the value exactly in my oils and paint over it. That "white" skirt, face doesn't look so dark now, eh?
In oil paint, I divide the jacket, skirt, etc. into general light and shadow. I cover up all the acrylic paint with oil.
I keep the light and shadow general so I can manipulate the patterns. I use a photographic reference as my guide but seldom feel bound by "reality" if I can think of something better (that looks believable).
I continue to work with just general light and shadow and start turning the background into "real paint." That is, I follow the underpainting as a guide and begin to add opaque colors.