Bodywork lasts far longer than it did a generation ago, and yet corrosion is still what finishes most cars that reach an advanced age. The improvements addressed the surfaces that are easy to protect, not the places where rust actually starts.
How corrosion actually proceeds
Steel corrodes when iron, oxygen and water are present together, and salt accelerates the process by making the water far more conductive.
The reaction produces a porous oxide that occupies more volume than the metal it replaced, so it lifts paint from underneath and traps moisture against the fresh surface.
That expansion is why rust spreads outward from a small point rather than staying where it began.
Why modern protection works so well
Bodies are now galvanised, dipped in electrophoretic primer that reaches into cavities, sealed at the seams and coated underneath.
Zinc coating protects in two ways, physically covering the steel and corroding preferentially where the surface is scratched.
Those measures largely eliminated the visible panel rust that used to appear within a few years, which is why old and new cars fail in different places.
Where it starts instead
Corrosion now begins where water enters and cannot leave. Box sections, sills, wheel arch lips and the seams between spot-welded panels all trap moisture by design.
Drain holes are provided but block with road debris, after which the cavity holds water permanently.
Because these areas are hidden, the damage is structural before there is anything to see from outside.
Why repairs often fail
Cutting out corroded metal and welding in a replacement destroys the factory coating around the repair and creates new seams that were never sealed in a dip tank.
Treatment applied to the outer surface leaves the inner face of the panel unprotected, which is where the corrosion usually restarts.
A repair that looks correct can therefore fail within a few years while the surrounding original metal remains sound.
What actually extends body life
Keeping drains clear does more than any coating, because water that leaves a cavity cannot corrode it.
Washing the underside after driving on salted roads removes the accelerant rather than the moisture, which is the more effective half of the problem to attack.
Cavity wax injected into box sections displaces water and creeps into seams, and it is the one treatment that reaches the surfaces where corrosion begins rather than the ones that are already protected.
Inspection matters as much as treatment, because the areas that fail are hidden ones and a body that looks sound from outside says nothing about what is happening inside a sill.