POR15 works great. I've used it on many restorations, and the AT. It won't chip at all providing it is applied properly to a reasonably clean surface. I have stuff that has been blasted by gravel for two years and still looks new.
First, remove all grease, oil, debris, etc. Use a good degreaser followed by brake cleaner and give everything a good scrub with a plastic bristle brush.
Then, if you have surface rust (e.g. not something that needs to be cut/welded), hit everything with Metal-Ready as directed.
Once prepared and cleaned, LET IT DRY. Stuff the car in an enclosed area with a heater and keep it above 50 degrees for a few days at least. Above 70 is better. Be careful that your fuel tank is not completely full as it will expand with the higher temperature.
Once dry, paint as you like. Again, I would not apply the paint below 50 degrees. Above 70 for application and curing is ideal. Be sure and wear gloves and cover up well, POR15 will stay with you for a while.
POR15 does not need to be primed, sealed, or anything else of that nature. It works better than most sealers on its own. You may want to use several coats in areas that will see a lot of road gunk.
Doing the underbody and fender wells is a very good way to keep those respective areas happy. I would personally not used rubberized paints as they can trap water if damaged and accelerate rust.
Specific to the AT, one thing you REALLY want to do to prevent rust is completely remove the rear hatch panels, rear seats and associated panels, and speaker boxes. That will allow you to closely inspect the rear wheel well seams, which are an extremely common trouble area for rust. Coat the backs of the seams via the access holes in the body and you'll be rust free for a good long while.
It's also wise to pay attention to the lower fender area directly behind the wheels. Again, you can access both sides through the access holes-- directly below the jack bracket is the area I'm talking about. Get a bunch of POR15 in there and you're unlikely to have rust issues.