ditto on what Rick said... around 20psi for me, unthinned. I have seen some modelers who use a 1:1 ratio of rubbing alcohol but haven't tried it so don't know how it works.. from looking at their models it works fine though.
Try cranking up the pressure and start with a mist and work up to heavier on some scratch plastic and see what happens! should do the trick for ya...
if you find you dont like the spraying thing, a wide soft brush can do the same trick when brushed slowly as not to create bubbles...
---edit---
back when I used my Aztec I sprayed it with a couple of drops of water to about an ounce of Future with good results...