After reading your application description, you have one of two problems. First is the pinholesI stated in my first post. When you spray primer on thick it tends to form pinholes. Apply another coat over the pinholes and you have trapped air. Your blisters form in heat, so I would still suspect this.
Another possible problem is gassing off. If the primer is not two part urethane, an application of several thick coats will take a few days to cure. One part paints dry by solvent evaporation whick slows considerably when the primer skins over. The primer drys from the outside in, which is very slow on thick coats.

Best of luck
Scott