I try to keep things in persepctive when it comes to frame rust living in the Northeast... it's inevitable here for the most part, unless one is super duper anal retentive about spraying down the undercarriage in the winter, which I'm not. I have better things to do.
What I have decided with our current vehicles (a 2005 corolla with 170,000 miles and a 2007 4runner with 94000 miles) to prevent frame rot is to keep them going using various methods (POR-15 undercoat and topcoat on the external surfaces, eastwood internal frame coating on the inside, Fluid film on all the nuts/bolt connections).
[When it's time for a replacement vehicle, I will buy new if still in the NE or a garage kept Land Cruiser from the south. As soon as I get it home, I will apply POR-15 and eastwood internal frame coating BEFORE any surface rust has an opportunity to grab hold.]
All in all though, I'm real happy with how the frame has held up to 8 consecutive PA winters (with no preventative actions before this summer) and a half dozen trips to the beach. It could certainly be a lot worse. Now that I have it treated, it should last another 20 trouble free years...
Assuming I don't trade it in for a Land cruiser in 5 years

arty: