If your car lives in a driveway, its winter exposure is what it drives through. If it lives on a Waterbury street, it also gets whatever the plough throws at it, and that is a heavier and stranger kind of exposure.
It is not the same as road spray
Driving through slush coats a car fairly evenly and mostly from underneath. A plough passing a parked car does something different: it throws a concentrated wall of salt-saturated snow horizontally, at speed, at one flank.
That has three consequences worth knowing.
It goes into the shuts. Horizontal impact drives slush into door gaps, around the fuel filler, into the boot seal and up behind the bumper covers, where ordinary road spray never reaches. Those places have drain paths designed for water running down, not for wet salt being fired in sideways.
It freezes in place. Packed against a cold panel, it does not run off. It sits and refreezes, so the contact time between chloride and metal is measured in days rather than minutes.
It is unevenly loaded. The street side of the car takes almost all of it. That is why a street-parked car often shows corrosion starting on one side well before the other, and people assume damage when it is simply which way the car was facing all winter.
The bit that actually corrodes
Not the paint. The paint is a decent barrier and salt on a sealed panel is a cosmetic problem.
What matters is the material forced into gaps and seams, where two pieces of metal meet and the coating is thinnest. Door bottoms, the lower edges of the tailgate, the sill seams and the pinch weld. Those are where cars rot, and horizontal plough spray puts salt directly into all of them.
What actually helps
Rinse the flanks and the shuts after a heavy week, not just the underside. Open the doors and the boot and look at the sills — if there is grey crust in the shut, that is what is sitting against the seam.
Get the underbody and cavities flushed properly at least once mid-winter, after a thaw, and again in spring. That is our undercarriage flush at $79, or part of a full detail at $275.
Keep the drains clear. Door and sill drain holes block with grit, and a blocked drain turns a door bottom into a container. It is a two-minute check with a piece of wire and it is the single most useful free thing on this page.
A sealant before winter does not stop corrosion, but it makes the flanks far easier to rinse clean, which means the salt actually comes off when you try.
The honest limit
None of this reverses corrosion that has started. If there is bubbling under the paint at a door bottom or a sill, that is rust working outward from underneath, and washing it will not touch it. We will say so rather than sell you a detail that makes it look better for a fortnight.