Here's a cool animated material test by Reynante. Read on to see how he did it.
This effect was achieved by mixing three main shaders: paint, metal, and rust.
From here, I controlled the transition/masking by animating the Color Ramp sliders that are plugged in a mix of procedural textures.
This is a currently tedious process and am looking at better ways of controlling the effect by using drivers. :)
Thanks so much to reddit user cmuadamson for the suggestion. I've had so much fun on this one. ;)
Model courtesy of Stanford University.