Great question, though I can't really help- I was wondering the same thing myself!
I'm interested in finding out if anyone uses the "One Shot" brand of signpainters paint? I heard that's good.
I've used standard latex paint for lettering on wood, chalkboards, gatorboard, foamboard, etc.
Depending on brand and sometimes color, 1 coat has worked. Red is such a pain in the butt to work with! ;)