Having left vehicles parked over grass for years I recognize the rusty underside.
POR 15 is great if you can get at all sides of each component, but the underside only shows herself to the road and your mechanic.
If the Sacramento area is as dry as I think it is you may have a hard time finding a rustproofing company.
If it was mine and I lived there I'd buy an air compressor attachment that sucks up liquids, an engine spray cleaner gun, ($9.00 Harbor Freight http://www.harborfreight.com/engine-cle ... 68290.html
) and spray a thin coating of transmission fluid underneath and in all the door panels; use the factory door plugs and drain holes for access. It won't need much and will drip a lot if you over do it, but yearly applications will stop the rust.
It will also creep into the body seams thereby helping stop the rot. A dry environment will certainly help halt it as well. I bet you use less than 2 quarts to do the whole thing.
Just remember to park it on the street if it starts to drip. Transmission fluid eats asphalt driveways.