With the masking process completed Brandon (far) is painting the subroadbed with a generous layer of water. Paul (near) is using a small sifter to dust the plaster and tempra pigment power mix (zip powder) onto the water. The method avoids the splatter than can occur if the zip powder is applied first and then is misted from a water bottle sprayer. Nearly all sprayers also emit droplets of water that act as artillery shells and splatter the plaster/pigment mud all over (especially on back drops). Water first eliminates nearly all the mud splatter.

When the zip powder hits the water, the mosture activates the plaster and tempra pigment creating a (hopefully) dirt colored mud. With the right amount of water the plaster dries with a rough texture (too much will create a slimy mud flow).

If you look carefully you'll see the blue painters tape dam on the layout edge that keeps the slime from dribbling down the fascia.