Why cars rust

my daily driver 1972 Coronet is originally from out of state ( don't know where from however ... ) , but has spent a better part of its 38 years in Ca.

the car's prior owner lived near LAX ( El Segundo , to be exact ) , which is a stone's throw away from salty beach air .
the upper "A" pillars where they meet the roof are rotted ; the lower "C" pillars are rotted ; the lower driver's side quarter panel is rotted ; the roof is pitted with rot ; and the windshield and backlight channels are rotted .

Why ?

damned vinyl tops !!!

A vinyl-covered roof is probably the single worst thing that can be applied to a car .
As much as I truly LOVE the Mod Tops , Gator Grain Tops , Paisley Top ( 1971 Imperial )
and the Dodge version of the Mod Top ( Peter Max-esque pattern ) , they were also the cause of rotted roofs , along with their ho-hum brothers ( white , black , blue , Gopher **** beige , etc. ).

Because of the extensive rot , and the necessary repairs , the roof won't be straight enough to leave uncovered ; another damned vinyl covering is , unfortunately , in order .
BUT I plan to rectify the problem by :
- Sealing the ever loving **** out of the metal
- Running a continuous bead of black sealer around the perimetre of the :
1.) windshield-to-trim
2.) windshield-trim-to-roof
3.) backlight-to-trim
4.) backlight-trim-to-roof
5.) "C" pillar and Sail Panel trim-to-roof-and-surrounding-panels

Hell , I'm even considering just spraying the roof with white-coloured bed liner stuff !

And , btw , this frickin' days-straight of rain are irritating the pizz outta me !! ALL of those rotted areas around the windshield and backlight are absolutely pissing and leaking water right-into my Coronet's interior !
And , no functioning heater / defogger / defroster = NO FUN .

"...it never rains in southern / California ..." MY *** !!