The flow was great with simply 20% thinner and no flow improver (I was afraid it would break the surface tension and ruin the contrast effect).
The fact that I did the entirety of leviathan in one batch probably helped limit the drying time in the airbrush, and prevented clogging the airbrush.
All 4 steps done with an airbrush: Prime Vallejo Black, then Zenithal Vallejo Grey, then one spray directly from above with Army painted white (any white should do). Finally, I just sprayed Skeleton Horde until it started pooling, at which point I stopped to avoid over spraying.