I use a product called Fluid Film (www.eurekafluidfilm.com). All the Volvos I've owned (except for the 944 I had briefly) have been pretty much soaked with this stuff. My Volvo mechanic got me interested in it when I bought my first Volvo from him in '01. Great stuff. A simple test is to leave a piece(s) of unpainted metal outside with one side (piece) coated in Fluid Film and watch for rust. It's not real great for coating the underside of a car because it can wash off under pressure, but it's great for the inside of panels where it pretty much sticks permently. The best way to apply it is to strip the interior (seats, carpet, door skins, etc) and spray with a pressurized gun. Works great, and after a few days of driving you'll start to see it creep out around body seams and places like that. Makes a bit of a mess and there's a little smell (I wouldn't call it a stink) inside for a few months, but it goes away after a while. Dirt sticks to it a bit and it stops creeping as quickly...
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Chris. Halifax N.S. '98 V90 > 206K km, '94 964 > 100K mi, '97 855 GLT > 265K km. SOLD: '91 745T and '91 745 NA (I miss both of them!).