Hi Doug
I used their paint on the Oneida & Western SD40-2 you saw at the PGE/BCR convention this past summer. Vallejo has two different types of paint. The Model Air stuff is made to airbrush right out the bottle (no thinning) and they have a thinner for their regular Model Color stuff. I have used both with very good results. I don't have a air gauge on my compressor, but I treated it the same way you would treat Polly Scale, about 1/3 thinner to paint. Drop me an e mail or phone call if you need any tips when you go to use it.
What are you painting?
Chris