RE: First airbrush job!
I use a 5mm needle and 60 psi to spray my liquid mask works fine. I use Bob Dively liquid mask, never had a single problem. Thinning i would try 9 to 1. 1 water to 9 masking. Might want to check to see if you can adjust your need further back in your airbrush also to help it spray, if you have a 3.5 or a 2 mm needle.