I have had nothing but problems with the MM Gray Primer. I threw it away.
On the other hand spray cans are great! Be sure to check the big box stores for the jumbo-economy sized ones.
Krylon, Plasticote, Rustoleum, American Painter - all make excellent primers. They cost less than the ones with the model manufacturer's logo. Be sure that the cans are room temperature or warmer and agitated well before use Spray light mist coats instead of a single gloppy coat.
[Oh, and by the way, did you know that Model Master paint is manufactured by RPM Industries, the same people who bring you Rustoleum?]
And to the original question -- there is no problem in airbrushing a light coat of flat white as a primer. Be sure that it is properly thinned and spray light coats.
Experiment and find what works best for you. There is no one-size-fits-all.