Agreed. I have just spent the last few days completely dismantling the back
section of my 1982/83 900S (normal 4-door sedan - not hatchback), and found
rust in interesting places due to water leaks, etc.

The car appears to have a lot of gunk which looks like BlueTac filling some
spaces that look like they'd be common water ingress points, and as I put
the back of the car together again I will be replacing all that gunk with
neutral-cure black silicone sealant since none of it is in any place where
it's directly visible against the white-painted bodywork.

I never realised until attempting the job that the black back section above
the bumper is actually a fancy aluminium extrusion! Neat for a car that's
20+ years old now and still running strong (I had to change the battery
recently too - last one was just over 12 months old and I am trying to work
out why it has failed)...

One problem I have found is that the sealing strip running around the metal
lip of the body which the rear boot closes down on is capturing and holding
water, especially along the top of the boot opening and at the left/right
corners. Is this a common place for rust to occur? I'll need to eventually
pull all the sealing strip off and give the whole of the metal lip a
thorough clean and rust treatment.

The other points where I've found a lot of surface rust are the front ends
of the left and right main chassis members, the metalwork holding the
radiator just behind the front bumper, and along the bottom lip and all
metal edges of the bonnet (presumably related to the heat in the engine
bay). I saw it when I had the bumper off for fitting of the foglights and
associated extra wiring (self-amalgamating butyl rubber tape is wonderful
stuff for sealing up cable looms coming out of the fuse/relay panel!), and
will need to have a good look next time I lift up the front of the car to
make sure there isn't any underneath.

Regards,

Craig.