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...

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

As a old timer once told me you can't fix it till you get there or otherwords start cleaning and see whats what, it might just be surface rust unless it's from washing it and leaving it wet for long periods. Most of the water in my teener came from a leaking rear window you might want to check that. I don't know of any reason why it can't be welded from the top though. I don't consider wire brushing and painting with por15 a permanent repair though. Por is ok but usually the rust isn't removed well enough to really stop it's progression.

Rust never sleeps. The moisture in the air is enough to keep it going.

A complete floor pan needs to be welded from the bottom as they overlap with the underside of the long.

If yours only needs patching, i don't see any reason why that can't be done from above ...

Take both seats out and get a good look at what's under that tar.
Andy

While technically true that it will keep rusting even without water, the rate of progression without standing water is exceptionally slow.

In my opinion without continual exposure to water, rust does not advance appreciably.

Also, I agree that rust on the floor pans is most often caused by rear window leaks, not by side window leaks.