This just happened to me this weekend as well. I looked under the car, and the floor pan for the rear driver side passenger floor panel is about to fall off. I'm planning on taking the area apart. I'll clean/ treat the rust then bolt/weld a new pice of metal in place. As for underneath the car I was thinking of bolting some LEXAN on.

I put some pics @ http://restoresb.blogspot.com/

the reason those cars 83,84,85,86,87 240s rust out the floorpans is from inside out.either windshield leaks,roof rack,am/fm ant.i live in south and have looked atplenty of so called creampuff 240s only to look under and laugh.unless you are half a structural engineer,the best way to fix floorpans is to cut or treat the metal the best you can[watch out around center support bearing]weld metal back into place and fiberglass over top of that,then unmdercoat from below.i realize that this is kind of vague so if you want i can email you with more precise ways of doing it,i have fixed several 240s.

I have a south Florida car that is now in California so it has never been through a salty season. The only rust is below the back windows and that was recently delt with with POR-15. I'd be very careful buying a north east car.
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Simon 80 240 300k 17 years. 'White Lightning'

I think '85 may have been the worst year ever for rust. I had a 244 which literally fell apart around the mechanicals when I lived on Long Island. It's hearsay, but I was told by my mechanic at the time (an exclusive Volvo and Saab guy) that for the 85's Volvo went to environmentally 'friendly' water-soluble undercoating, which actually absorbed and retained moisture. Given the Swede's proclivity for environmentalism, combined with the fact that the 240 glove box door is held on with candle wax (true, right guys?), I find this entirely plausible. I believe from '86 on the unibodies are galvanized and Volvo's ore one of the LEAST rust-prone vehicles on the road.

I've had 7 bricks from a '63 122 to a '93 240 Classic, and the only one with significant rust problems was that '85. Floors finally DID give out - a great runner though. Probably didnt' help that I lived in a salt water environment and the road crews heavily dosed the streets at the first sign of freezing rain.

Too late for you (or anyone with an '85 in cold climates) to prevent damage, just do your best.

Good Luck!

Rust was a major problem with my 85 245,,which bit the dust
at 356k miles. Never used a drop of oil, but I did have to
remove the seats and put in some sheet metal. Absolutely the
best car I've ever had. In fact, if I'd had half-a-bay of space,
I'd still have most of it.
Best of luck and Happy Brickin'!