i bought 2 quarts of Por 15, a quart of the Marine Clean, and a quart of the Metal Ready. now, i would definitely recommend you do the POR 15 treatment before doing any undercoating. i had mine undercoated a few years back and now i'm trying to take it off as much as possible to treat with the POR 15, works a lot better the other way around. as for the POR 15 amount, i haven't even touched the second quart yet and the first quart is more than half full still; after 3 coats on my old calipers, a few misc car parts, and 3 coats on my 1st gen roll bar. a little bit goes a long way, but you may need to buy more of the Marine Clean and Metal Ready. however, once you open it, it will start to solidify slowly even if you do as the directions say and put some plastic wrap between the can and lid and keep it in a cool place. i came back after a week of not using it and there was a "crust" on the top inside the can. i would say you could probably get away with one quart for your undercarriage. as for how to use it...
get off as much of the loose rust as possible with a flat screwdriver, wire brush, or some gritty sandpaper. then mix the Marine clean 50/50 (or less) with warm water (use gloves!!!) and clean everything you'll be painting with the marine clean and rinse with water. maybe dry it with a towel, let the heat evaporate the water, or use compressed air to blow it dry. the bottle says to use the Metal Ready straight, and keep parts "wet" with the product for at least 10-15 minutes, or longer if they are heavily rusted. a spray bottle would probably be best for this under the vehicle. then rinse with clean water, dry as mentioned above. then it's time to lay some tarps down (if you don't want your driveway/garage/street covered in POR 15. might be a good idea to cover yourself thoroughly too, and definitely wear gloves or you will be POR 15'd for a month. now, crack open your quart and start painting! do a light coat, wait a few hours, do another light coat, wait a few hours, and then do another light coat and be done. don't do a heavy coat or you will have lots of drips and sags and it will take longer to dry. i'm not sure about moving parts or threaded bolts etc with POR 15 put on them, it dries pretty solid so if you paint it on something make sure it's not a moving part or a part that you will have to replace down the road or it will be harder to remove.
and, no you don't have to disassemble your entire car to use POR 15.
it's quite a lot of steps and waiting and messy, but if you want it sealed for a long time you'll have to struggle a little in the short term for long term protection.
hope this helps you
