I actually do more of a layering technique with opage and semi opague colors. Once I have my drawing and underpainting established I replace a tone with the local color of the object or portion of the object and manipulate the color from there. I use a combination of layering, scumbling and at times glazes.
Actually, from the perspective of the painter the painting is simply evolving. Time only moves forward and each revision begins the painting anew. It is only through outside eyes (or the effect of videotaping) does the painting experience shift from the "painter's now' into the viewers collective experience of present, past and all the increments in between. Simply put, while in the bliss of painting I am unaware of the outside reality of multiple paintings. Ahh...the Zen of Abstract Painting.
Regarding the oil pastels, generally they are the last marks I make on the painting. Simply because the oil pastels would repel additional layers of acrylic paint. Actually that would not be much of a problem I use a minimum acount of pastels. I also try to use pastels that are less oil content.