Wetting the floor keeps the over spray and you from sticking to it, I wet it then use a squeegee to push out the excess, you want it damp but not dripping. Yes temperature and humidity add to the mix. I try to paint when the humidity levels are low, late spring, late summer are best, low humidity low bugs. I also prep and mask the car the evening before I shoot it, then paint it the next morning when dust, bugs, etc are low.