Ah, trying to airbrush jawsets with white teeth and pink gums. Much easier said than done. First, the paint has to stick. Second, it has to go where you want it and not where you don't.
If there is even a trace of release on the cast forget it, the paint will not stick. The good news is, resin cast into silicone rubber doesn't really need release! Yay! Unfortunately the paint doesn't like to stick to bare resin either, and according to the saints at Smooth On Technical Support there are very few primers that will help. One is the Bulldog Adhesion Promoter shown here. The other is Plasticote Sandable Primer Enamel.
The areas where paint is not wanted need to be masked. What to use? Liquid frisket came first to mind, here applied with silicone shaper tools. (The frisket would quickly ruin a regular paintbrush.) For me, this was very tricky and took several hours to apply. I only attempted masking the teeth with the frisket, as I was completely daunted by the much larger surface area of the gums.
Below, the gums airbrushed and the frisket removed.
Not too bad, except that the paint was still fairly fragile and the frisket would pull bits of it off around the gumline. I was not convinced this would be the go to method.
Below, various airbrushing attempts, in various states of completion.A big fat pile of frustration.
Some of these are solid pink plastic, with the teeth airbrushed white; some of these are white plastic with the gums airbrushed pink. Ultimately, though, I was not happy with the durability of the airbrushed paint- even with the primer, and even sealed with a topcoat afterwards, it was too easy to scratch and damage. Maybe there is a method I hadn't discovered yet to make the paint more durable, but for the moment anyway I am SO DONE airbrushing these things for sure. On to plan B, attempting to cast in two colors of plastic!