Washing your car in a Massachusetts winter

MassDOT's liquid brine and magnesium chloride keep a car wet and salty for longer than dry rock salt does, and the real damage happens underneath, where your paint (and any coating on it) never reaches.

I'm based in Wareham, on Buzzards Bay. Here's what I'd tell any owner in this part of the state.

More than rock salt. MassDOT lists five winter materials: rock salt, liquid salt brine, liquid magnesium chloride, sand, and a rock salt and calcium chloride pre-mix.

Two of them matter most for your car:

• Brine is salt dissolved in water at roughly 23 percent. It’s sprayed on the pavement before a storm so snow can’t bond to the road.
• Magnesium chloride is sprayed onto granular salt as it leaves the spreader, so the salt sticks to the road instead of bouncing off.

That first point catches people out: the car is driving through chloride before there's anything white on the ground.

Salt on its own doesn't eat metal. Chloride dissolves into the film of water already on the car, makes that water far more conductive, and breaks down the thin oxide layer that otherwise protects steel. That corrosion process needs liquid water to run. Dry salt crystals on dry metal do comparatively little.

Plain rock salt residue dries out into crystals in moderately dry air. Magnesium and calcium chloride are much more hygroscopic: they pull moisture out of the air and stay liquid, or turn liquid again on a damp day, at humidity where plain salt would be dry. The car spends more hours wet.

The research genuinely disagrees on which is more corrosive overall; Colorado DOT testing got opposite answers depending on whether parts were cycled wet-dry or submerged. The takeaway doesn't depend on who's right: whatever's on the car stays active longer than it looks.

Winter rewards frequency over thoroughness. Wash the whole vehicle, undercarriage included, whenever temperatures allow, and wash after each significant storm cycle. Doing that consistently matters more than doing any single wash perfectly.

Underneath. Two things reach the underside of a car in winter: slush thrown up by its own tyres and the car ahead, and a fine aerosol of brine that drifts into places nothing splashes. That aerosol gets into box sections through factory drain holes, behind wheel arch liners, into seams and pinch welds, and into the bottoms of doors and rocker panels. Those tight, damp crevices are where corrosion moves fastest.

AAA's list of what de-icers damage is worth reading literally: brake lines, fuel tanks, exhaust systems and electrical connections. Those are unpainted parts below the bodywork, and they're what winter washing is really for.

Yes, as long as you know what it can and can't do. An undercarriage wash reaches the surfaces the spray can see: the floor pan, exhaust, subframe faces, control arms and the outside of the frame rails. It doesn't reach inside sealed or semi-enclosed cavities, and it doesn't chemically neutralise chloride. Highway research on winter maintenance fleets found soap and water alone weren't enough to remove residual salt.

Three details make it work better:

• Volume and dwell beat pressure. Very high pressure can push water past seals and into connectors, and can strip existing underbody coatings.
• Drain holes have to be clear, or washing just adds trapped water.
• Drive for a while afterwards. It helps clear standing water from cavities that won’t air-dry on their own.

Each trades one risk for another. Touchless can't drag grit across the paint, but it leans on stronger chemistry and tends to leave a film on a really dirty car; on a coated car, strongly alkaline pre-soaks are a slow chemical wear route. Brush and cloth washes clean heavy soil better but can drag grit from the car ahead across yours. Neither is risk-free.

The technique is the same as any other time of year:

No. A coating does three real things against road salt: brine and grime stick to it less, so rinsing removes more; it takes chemical exposure in place of the clear coat; and it reduces the staining salt residue leaves on paint.

But a coating only exists on the painted surfaces it was applied to. Frame rails, brake and fuel lines, exhaust and the insides of doors and rockers aren't coated and gain nothing. A coated car and an uncoated car have the same underbody exposure. A coating over a rock chip that reaches metal doesn't seal the chip, either. It makes washing easier, not unnecessary. The rest of what a coating does and doesn't do is in my ceramic coating guide.

Then winter isn't your only salt season. Breaking waves put seawater into the air, and deposition drops off steeply in the first few hundred metres from shore. Fog, dew and onshore wind deposit salt and wet it, while rain mostly rinses, so a run of still, foggy days can be harder on a car than a storm. Putting a car away wet and salted in a closed, humid garage keeps it wet longer; give it a drive after a wash before you shut the door.

• Waiting for the “big wash” instead of washing after each storm cycle.
• Skipping the underside, or blasting seams and connectors at close range.
• Letting it drip-dry, then shutting it in a closed garage.
• Stopping when the plows stop. AAA is specific that leftover deposits keep corroding year-round, so do one deliberate underbody flush in early spring.

Look where the brine settles, not at the hood. The bottoms of the doors and rocker panels, the wheel arches, and the seams are the crevices where corrosion advances fastest. If road film is building up there between washes, wash more often, and include the underside every time.

• Before winter starts. Decontamination comes first, because a season of iron and road film holds moisture and salt against the paint, then protection goes on (here’s what a clay bar does). Every product has a temperature window and a cure period, and once the car is being driven through brine that window is hard to protect.
• You have chips down to primer or metal. AAA’s advice is to repair them before the season. A chip packed with wet brine for four months behaves very differently from the same chip in July. That’s a touch-up or body shop job, not a detail.
• You want a coating before winter. An unheated space in a New England January is below every coating manufacturer’s minimum temperature, and the coating keeps curing for days afterwards, when it realistically shouldn’t be driven on brined roads. Fall is the window for a ceramic coating.
• You want rust-proofing on the underbody. That’s a separate product category on separate surfaces, and not a service I offer.