You are right about antirust zeebart tar type of coating.

Here in eastern Canada Montreal area antirust treatments are popular. Many types are availlable.

Metropolitain for one injects under pressure a liquid parafin type. They inject inside doors, in rocker panels, inside trunk and fenders, all around hood and trunk cover, inside the frame structure.

The car is after this driven outside to drain 30 minutes on a liquid recovery floor. They garanty no rust if treatment is done every year.

I prefer the heavy liquid parafin type. It does not drip but it spreads slowly with time on ereas not covered. It is applied with a jet of 150 psi compressed air, the spray gun is about 1 quart.

Nothing is perfect, before I do the treatment by October or November I have to scrape inside rear fenders to remove a thick coat of black dirt that is sand and whatever dust absorbed in that greasey stuff. And it is no fun to work under the car, repair something like clutch adjustment, fluid level check in transmission etc.

But it does the work, I hate so much repair rust holes. My 1982 has 37 winters in road salt.