I assemble, base and prime my models off the sprue with spray primer. If they even look at me funny, I pin them to the base and paint the pin black. I used to prime with Games Workshop primer, but found it too expensive for what it did. I'll update with the brand I use later.
Regarding spray priming: I (try to) spray in temperatures around 70 Fahrenheit/20 Celsius with 70 percent humidity. I've sprayed in cooler and hotter temps, but I try not to take risks. The real thing to be careful of is humidity: spraying when it's "hazy, hot and humid" is asking for the mini to get a thin coat of fuzz. Humidity is a function of the amount of water in the air so don't spray when it's raining.
The best way to spray primer is outside, during a warm, clear day. Place the minis on a flat surface (I use a cardboard tray I buy with my beverages), spray one side before moving onto the next side then move the tray inside to dry.
Hope this helps!