Unfortunately, no car is oblivious to rust, at least I haven't seen one yet. 700s will rot eventually and there are many factors that determine when. Here in Nova Scotia (lots of road salt in the winter) I've seen 15 year old local winter driven 700s with very little rust and this summer I saw a '89 744 that was VERY rotten underneath. The frame rails were rotten as you discribe, but that was just the beginning; the floor was rotten, the bottom of the rockers, the trunk floor and a couple other places. It was really a shame as the rest of the body (doors, etc) looked great, the paint even appeared to be original and in great shape. If I see local winter driven 700 in that age range that's in good shape, usually it has been rustproofed with some effective product such as fluid film that has fended off the rust. 700s are much more rust resistant than most other cars though, their bodys are very robust especially if well maintained. Your '86 may be repairable, it depends on the extent of the rust. I'd say you'll have to consult an experienced welder/body repair person, good luck, I'd hate to see another Volvo taken off the roads!
--
Chris. Halifax N.S. '98 V90 > 195K km, '91 745T > 300K km (FOR SALE) and '91 745 NA > 409K km.