When we built firetrucks in Canada (and Finland & Germany) , many customers (especially in Quebec) wanted them undercoated aka "rust proofed" (Rusty Jones, Ziebart & etc) ... we even pre-drill holes all over the body, compartment doors and etc. and pumped that crap everywhere.
In most case the undercoating caused more problems (over time) than it solved. Example, missed spots, cracking, chipping & etc ... therefore holding in the salt spray to rust even more later.
IF ... you could completely seal it EVERYTHING ... without screwing up other items, like springs, fuel lines and other misc parts ... in theory it works ... but in real life it didn't.
Best way was to clean vehicles after every time in the salt spray ... period.
PS ....And if you had a personal car that was in the salt all the time ... it was best to leave it outside in the cold at nights and not in a warm garage to rust.
Montreal used to have the worst salt problems that I have ever seen. Now, they use more sand and non corrosive liquid chemicals ... but years ago (late 1970s & early 1980s) ... it was PURE SALT ...