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.