Brush-on primer? Or: when is it too cold to spray?

I live in Pennsylvania, and we're starting to get our first real winter weather. It looks like Thursday and Friday will be sunny and maybe get as high as 50 degrees, but I'm wondering if any other painters living in colder areas have more experience with this. Has anyone used GW's Imperial Primer or an equivalent product, and if so, do you have any tips for using it properly? And how cold would you consider too cold to use a spray primer? I've tried to find a simple number, but recommendations are all over the place; humidity seems to be a bigger factor, but I'm not sure I want to risk it until I know what's up.

Yep. I have imperial primer and thought poorly of it, heavily favoring Vallejo surface primer brush-on line (I'm partial to gray), but have just found out that I wasn't using it properly and am going to try it out again.

I find my stuff is usually ok being in the UK though we sometimes don't hit those extreme temperatures. If it gets too cold for me to spray outside I prime with an airbrush indoors, I almost never prime by brush anymore as it is too inconsistent.

Here in Brazil we don´t have problems with cold weather, but with rains, humidity may ruin the spray and your outside airbrush. The real problem is the heat, I need to use icepallete to avoid the paint drying too soon on the pallete!

Thanks for the advice. Looks like I'm better off just priming as much as I can before the real cold settles in; it'll be humid or rainy for the rest of the week but the middle of next week should give me one (hopefully not last) chance to get some stuff sprayed. I need the time to clean up some poor modeling work from 15+ years ago and finish assembling a few more models...