Thank you @mnallica! I have a progress slide show of a similar piece here: http://www.gric.at/gallery/bild186_m.htm

It's painted in several thin layers of dry acrylics. The details are cross-hatched in layers. I also use the airbrush for glazings, shadows and highlights. This gives the painting a depth and transparency which I can't achieve with the alla-prima technique. In the past I've also used oils in the final layers to get more depth in the glazings and softer, more vivid colors.