Pro-car rust

I'm gonna ask a question, maybe a stupid one.....checked with the search function and didn't find anything that I could see that would answer my query to my satisfaction. Here it is:

Why do pro-cars seem to rust worse than factory-built cars or is it just me? For example, in the Sad Pro-Car threads right near the beginning there is a 75-76 ambulance [don't remember if it's a M+M or some other coach builder] sitting in someone's driveway with the word 'Kimball' on the roof and the front end is rusted all to hell, even the headlight doors and the hood!! I don't seem to remember this happening with factory cars when I've roamed around various junkyards......any ideas?? Or correct me if I'm wrong....

When it comes to factory sheet metal... i.e. the fenders, hood, etc.. everything forward of the windshield, I believe that some year Cadillacs have more of a rust problem than other years. I don't believe that the pro cars rusted in this area any worse than the standard production cars.
Now, if you want to know why the bodies of the professional cars rusted, it is a very simple answer. The body builders didn't paint the raw steel on the inside. By the 1970's, GM was already using immersion bath priming of the vehicles, so all surfaces of the car body were treated. The body builders didn't have this level of sophistication, so they just ignored the interior metal parts, when it came to rust proofing. Miller Meteor, to their credit, started to Ziebart the doors and quarter panels, so that did help to preserve the cars to some extent. Once these cars are no longer being used, the moisture builds in the inside, and they start to rust, from the inside to the outside. My 1969 MM Caddy came from the high desert area of Oregon, and is very solid. Luckily, the rust to the interior is only surface rust, however, there are a few pieces that rusted badly, and will need to be replaced. Luckily I have a replacement already from a car that I helped to part out.

Additional factors involved how the cars were used, cared for, and stored over their service life. I have seen many high-mileage cars (yes, even here in the Salt Belt) that are virtually rust-free because they were never left to sit in a heated garage with gunk hanging on them. On the flip side, I've seen countless examples of low-mileage cars with beautiful interiors and rusted-out exteriors because they were used sparingly but allowed to return to heated garages after winter use, not being washed again until just prior to the next use.

Nowadays, it is common for funeral homes (in the colder climates) to have attached, heated garages. Years ago, not so. The cars were often kept in detatched, unheated garages. Block heaters were used to aid with cold weather starting and space heaters were sometimes used to keep the patient compartment of an ambulance or combination warm. I've talked to several of the fellows who sold these cars back in the days of unheated garages and they've all told me that trade-ins were often in much better shape when they weren't kept in heated garages.

rust like humidity is all relative. what to one man is rusted out hulk to another is not that bad. but I have not noticed around here that the front end sheet steel rust any faster on a pro car then a standard caddy. the leading edge of the hoods were a problem place due to the way they spotted in the inner structured. if the car is in a place were it gets a lot f dirt blown in then they rust there a lot faster. same as the lower rear edge of the fenders. natural traps that collect stuff that holds moister. like Tony said those people that flushed out there cars keep them those that did not lost them.