In fact, "rustproofing" the car, at this point will only encourgage further rusting.

First of all, "rustproofing" is a marketing term. Just because something has been "rustproofed" doesn't guarantee that it won't rust. Rustproofing simply delays the onset of rust.

If your car was bare metal (no paint), it would develop rust fairly quickly.

Your's car's paint acts as a skin to protect that bare metal from water and corrosive elements which would cause rust. But paint is pretty delicate, so once it is compromised, the underlying metal rusts just as if the car was bare metal.

If a car is "rustproofed" when new, the rustproofing acts like a second, much better, skin to protect the paint and the bare metal from the water and corrosive elements.

But once a car has been used, especially through a winter with salt on the roads, the paint has already been compromised in thousands of microscopic areas and covering those compromised areas with "rustproofing" only seals those areas so that they rust underneath the rustproofing.

I would not rustproof this car.