This is a completed underpainting:
It looks deceivingly finished, but that's only the digital photo. Up close, its still pretty "blocky":
This constituted about 3 passes over the entire surface. So a cross-section of any given area would have at least 3 thickly applied, opaque layers (of course-ground oil paint with alkyd medium to speed drying).
I'll now begin the more arduous over-painting in, at least, two layers, increasing the medium* content so the transparency increases. Then a final glazing layer or two with high-medium content. I then retouch white highlights to bring them back to the highest possible key value (as adding medium often dullens the brightness of the whites). I also tend to skew my pigment values toward the cool end of the spectrum knowing that any added medium tends to warm up colors (or skew them slightly to the yellower end of the spectrum).
*"Medium" is a term for various liquids added to the thick oil paints. From the tube, the paints contain only ground pigment and linseed (or safflower or walnut) oil as a vehicle for the pigment. I add medium to the paint to thin it, make it more pliable and in some cases, more transparent, and also to speed the drying time. The transparency allows light to pass through and reflect off of the paint layer or layers below. For medium, I tend to use Gamblin Galkyd, W&N Liquin, and most commonly Gamblin Neomegilp. And in all cases, I use as little as possible.