I'm going to put some kind of undercoating on my stang, mostly just for cosmetic reasons cause it never gets driven in the winter and rarely in the rain. I've heard that some undercoatings are no good because sometime tiny bits moisture can get in between the coating and the body and slowly rust it away. Just looking for the best/easiest stuff to do myself.
Por15 is the way to go. I'm using it on the entire undercarriage. You can use it on already rusted metal or on clean metal. They make this stuff called metal prep that you can use before you brush on the Por15.
I sprayed POR15 gloss black on mine. You can also brush it. I recommend brushing it because it makes a real mess and sticks to everything around it. It WILL NOT COME OFF METAL, DIRT, RUST, or YOUR HANDS!!! DO NOT get is on your hands. Wear a resperator, and safety googles and industrial gloves.
then put a big ass fan blowing on the bottom of the car and use a high pressure air nozzle to get any water that might be hiding in seams or anywhere else letting it air dry for a while really isn't going to hrt at all.
I'm not gonna lie it's time consumming and costly at the same time and I'm sure it would be a pita w/o a lift and a decent size compressor. But damn after seeign this I will do my next car the same way for sure.