What are these "rear valence flaps" that you guys keep speaking of. I had pondered fiberglass and bondo for my quarter panel. It is convenient being able to check the tread on the spare from the outside though...
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1981 Volvo 244 GL w/ 225K Original B21F and 3spd. auto

The later years of 200s have a sort of plastic wrapper with integral mudguards that go behind the wheelwells in the rear, the proper term being apparently a "valence." So the idea is that you salvage the kit to attach these to your car, and I suppose do a rough job of fixing whatever horrors are to be concealed beneath. I considered doing the same thing to my car, but sheer cheapness led me to use a combination of riveted aluminum, bondo, and hardware cloth to "fix" the mess on my 240. Aluminum went inside the wheel arch, hardware cloth and bondo where sculpting was required. It was amazingly successful, and was still looking good and rust free years later when I gave away the car. In fact I'm going to have to stop telling about this marvel of shadetree engineering on this board or people are going to get sick of hearing about it...

1986 and up 240s have plastic valence panels or skirts or whatever you care to call them, all around the underside of the trunk, rear bumper, and sides of the spare tire well areas.
It's a one-piece plastic stone shield designed to keep the bodywork from chipping, which is what happened to yours- the chips in the paint let water + salt work on the metal, rotting it out. That plus the newer style mudflaps will hide a lot of the rust repair you'll have to do.
Your best bet is to probably find a wrecked newer one, 90-93 and cut the parts you need out of it. Junkyards usually have air chisels or gas-powered chop saws that can carve out any sheet metal you need. Sawzalls work nicely too.
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE: 625K total