NEVER NEVER spray on a humid day. Got that....... NEVER.
It is always humid here. The can needs to be the same temperature as the air so as not to condense as much moisture.When you have to move the miniatures from the AC to outside to spray prime, wait a few minutes for the miniatures to reach the outside air temperature. It is about 78F in the house but well over 92F outside. Probably 90% humidity.That will frost the primer. Keep both the miniatures and the spray can in the shade.Leaving Fine cast in the sun down here will make you very unhappy.
Dew point
The dew point is the temperature at which the water vapor in a sample of air at constant barometric pressure condenses into liquid water at the same rate at which it evaporates. At temperatures below the dew point, water will leave the air. The condensed water is called dew when it forms on a solid surface. Paint frost is caused by condensed water.The water condenses around the paint particles. I am sure someone can provide a better explanation. Reduce the difference between the air temperature and the spray can temperature will reduce the problem.
Warming the miniatures is not something I have tried. It is rarely cool enough for spay paint not to work properly here.
Warning about primer. Take care with sandable primer some of the autobody primers must be sanded. Try them first. The Flat black brand I use is thin paint and works well but requires several thin coats. The gray primer can be sanded. I use it for small autobody repairs too.
Photo of the Walmart brand green I use to base coat my home treated litchin. Get the flat black or flat white. The Gray primer can be sanded but works fine for miniatures.