After using a bunch of different acrylic and lacquer matte varnishes, I've finally found the best formula.
The Mix:
-1 part Liquitex Flow-Aid
-20 parts distilled water
-40 parts Vallejo *Polyurethane* Matte Varnish (Vallejo sells a few different matte varnishes. The PolyU is the newest one).
How to shoot:
-20-25 psi, 4-5 inches away from the model.
This mix dries "Dead Flat" on a surface treated with a gloss coat (e.g., Future, MM enamel) in 2-3 minutes and has yet to clog an AB nozzle or cause a dry tip. Plus, it doesn't cause hazy spots on the model. Easily cleaned with water, Simple Green or Windex.