Testors white enamel

It tends to age poorly in the bottle, once opened. But if new, make sure that it's thoroughly stirred and then it usually airbrushes with no problems. If you're doing a gloss finish, put down a flat white coat first to act as a primer. 

If completely mixed and thinned reasonably correct, it should airbrush fine, go on the surface fine, flow out and level, all very well..For myself and my airbrushes, at 25psi. The problem with it and many whites is opacity. That is to say, ya it went on great but didn't cover well. The answer to that has been brought up already, put on a flat white or as I would do, use white primer.

I don't use Testors enamels much anymore but when I do I now use hardware store lacquer thinner in them for thinning. The flow out and gloss is perfect, thinned someplace between 40% to 50-50 thinner. 60/40 paint to thinner is a very good starting point and it may suffice entirely.

I have no suggestions if your paint is old but to get a new bottle.

Paints tend to polymerize, or in other terms, the molecules link together in long chains. This gives the paint its strength and adhesion. Older paints may already be going through this process in the bottle and prevent them from spraying through your airbrush. I've thrown out more paints that have turned to a blob of rubber in the bottom of the jar. No amount of thinnig seems to bring this paint back to life. This always happens to me when it's late at night and all the hobby shops are closed.

This is from a guy who is painting all his automotive projects from the little bottles of Testors paints.

oldermodelguy

The flow out and gloss is perfect, thinned someplace between 40% to 50-50 thinner. 60/40 paint to thinner is a very good starting point and it may suffice entirely.

That has been my experience as well with the Testors enamel.  3 parts paint to 2 parts thinner seems to be the sweet spot for good flow and coverage.