It dries soft, kind of like a sticky upholstery cement. It does dry out eventually, but it takes a long time to do so. My Nissan is done with that stuff from the factory, and it seems to work pretty well. It is basically what they use when you pay to have a car rustproofed by a professional. It's available in a spray can, or a gun spray mix, but the gun spray stuff requires a special, expensive gun, so I just do spray cans. Mine I think was from 3M. Your local Finishmaster, or whatever you have in your area, should have the stuff or be able to order it. It may also be available from Eastwood.
One other thing I plan on doing with my car, is after it is "dried in" ( windows, weatherstripping, etc.) installed, I plan to sit in it during a rainstorm and watch for any water before I put in the carpet and sound deadening. This way, I can see if there are any leaks and fix them before it goes together. I like butyl acetate strip caulk for this as it lets me seal up any firewall leaks pretty easily. Commonly, the car will leak at the A/C box, steering column, and wiring bulkhead connector and grommets. I also did the (hated) fiberglass resin and mat coating of the floors. My floors were not that bad, and I couldn't afford POR 15, so I went this route to waterproof them and the rust repairs I had made to them. I did top and bottom ( 'glassing upside down is a b*tch!) so as to seal them as well as I could and add a bit of structural reinforcement where the floor had pinholes. I would have preferred new pans, but I feel this will last for many years if the car is watertight. Besides, I don't have the time to do it right right now.