seastallion53
First, Model Air should be able to be sprayed without thinning, for various reasons we like to thin it but meanwhile it will probably go through your airbrush fine. I propose you do a test shoot onto something plastic, a spoon, spare part or even a spru from the kit you are painting but something plastic.. . From that compare your dry time. It should be dry to the touch in minutes and handleable the next day.
Most paints should have primer under them, most acrylics really need it though. You will run into adhesion trouble and wear through problems enough, never mind with no primer. Primer really matters with most acrylic paints. Nuff said.
By the way, if it dries fine without thinner I think you know the answer regarding the thinner you used..