It's funny because I just watched a bunch of youtube video's on older cars from the 30's all the way up to the 70's. One video that I am sure some of you here have seen is the one regarding the 62 Cadillac promitional video and how it explains it's quality, value and performance.
Some of the old vids I saw spoke about rust proofing and how it was done. Bodies being fully dipped in electrostatic primer so that every nook and cranny was protected from rusting. I mean the engineers knew about rust prevention forever it seems, but yet nobody can explain the ferocity of rusty cars from the 30's all the way up to 70's. Even the best efforts in those days just wasn't enough it seemed. It's crazy to think it's taken so long up until the last 10-20 years where automakers have finally been able to prevent and even stop rust in it's tracks.
I also feel that in the 70's, the metal quality probably did get worse in terms of corrosion resistance even quality compared to the cars built in the 50's and 60's. Poor drainage, and styling choices for many cars during this time made it pretty much impossible to build a completely rust proof vehicle.
It's very obvious that today, you really don't have to worry about rust anymore, beside for the people that live in the mid west and east coasts where the underbodies are still prone to severe corrosion. The modern car lacks unique shapes to trap water, plus cars today are designed with drainage in mind and zero places for water to be trapped in.
Climate matters a lot here, many people think that cars from California are rust free and perfect, this is far from the truth. My 64 Cadillac is an original CA car purchased from an L.A. area Cad dealership back in the day, but the owner lived in Huntington Beach where the salt in the air has cause surface rust, and rust blistering over many sections of the car including the underbody parts. Even where I live in CA, we still get humidity at night, so the dew point moisture that sits on top of the car from the late evening hours to early mornings can cause rusting as well.
The only CA cars that are truly not affected by rust is if they were located more inland or further east towards the mountains and desert regions of the state. Even being 10 miles away from the ocean in CA can still cause damage or further increase rusting problems on a classic.
I knew a guy that lived in Pacific Beach in San Diego, he owned a 60 Mercury Meteor. That car had so much rust all over the body that it looked like it had been from Chicago went to Detroit, and headed back to California, that is how much rust there was on the Merc and his car was also a original CA car.
Rust sucks.