64, the more humid it is the quicker it will set up but under normal conditions it should be ok. POR-15 will go a long way and I have used it quite a bit on rust. As far as I'm concerned it can't be beat. I can tell you from experience that I roughed new floor pans on one of my 54's and it did not stick and I used 36 grit on a grinder. Like I said earlier pour out what you need and have at it. Just make sure the can and lid are 100% clean or you will not get the lid off. Use lacquer thinner to clean it/them. Your amount of humidity shouldn't be a problem.
One other product I sell is Fluid Film. It comes in a spray can or regular paint cans that can be sprayed with it's own gun. This stuff works like gangbusters. Spray it on anything that squeaks,rusts or sticks. Here in Pittsburgh I spray the whole underside of my cars once or twice a year and it works to stop existing or new rust. Go online to Fluid Film and watch a couple of videos on it.