I used the Gac 500 and transparent airbrush extender today....sprayed it through the airbrush of course...and we have a winner. It looks like a brand new scratchboard
The correct ratio is 2 parts Golden Transparent airbrush extender and 1 part Golden Gac 500. I took two pics below, the one at an angle is to show the sheen it leaves which looks like the original scratchboard surface. Photographing it was easy, no glare (I put glare on it on purpose)
Note: when you mix this stuff together it is white but dries and sprays clear.