Davout,
I live on Guam, which pretty much defines humidity. I have never had a problem priming minis, terrain, etc. In fact yesterday I was priming in between monsoon rain storms and it is all good. I always use Rustoleum primer. It is cheap, available, and gets the job done. Clear coating is another story. I have been a victim of the dreaded frost. I have switched to hand varnishing my minis and terrain, painting indoors under aircon. I use Winsor and Newton Galleria Matt Varnish on the advice of our own Captain Blood and it works very well. Best of luck and don't be too distracted from your wargaming by the awesome music scene in Austin.
LB