We've had success using the third Mindstorms motor to drive the sprayer. Here's a photo of it and we'll put together a movie of it in action. We are currently using the following sequence:

1. A timed spray to draw the solution into the tubing.
2. A pulsed spray to place metal precursor solution on the glass plate.
3. A short timed spray to draw air into the tubing. This prevents contamination of the rinse solution.
4. A long timed spray of dilute HNO3 solution
5. A short timed spray to draw air into the tubing, just like step 3.
6. Repeat 1–4.

Sprayermotor