Rust and Floorpans, Does it really spread with no water? Options

[Gudhjem]

When I removed by seats to repair my wiring harness last week I casually picked at a piece of tar that seemed to be lifting up from the floorpan... (IMG:style_emoticons/default/sad.gif)

Yep, I'd long thought my self lucky as far as rust goes, none to speak of in the hell hole, etc, but I am now disabused of that notion. My floorpan, at least on the driver's side, looks to have lots of rust under the tar, no doubt a product of parking outside and window leaks the car had to endure for 5 years. I haven't got up enough courage yet to poke around and learn if it's rusted through anywhere - but I have to admit it might be.

I have two questions:
1. Is it really the case that the rust will continue to destroy my car even if I never let it get wet? Can humidity alone do that? I live in mostly dry California and the car is now garaged.

2. If it turns out that the rust is bad enough that the clean/Por15 strategy wont work, is it possible to weld in floorpans without welding from underneath the car? Seems that every thread I can find about floorpan replacement involves cars on rotiserries or welding gurus that aren't daunted by overheld welds. I'm a decent TIG welder, but not sure I want to tackle that.

--Steve