The best rust prevention sprays for road salt

Why soft beats hard

There are two families of underbody product and they behave in opposite ways over time.

A hard rubberized undercoating sprays on thick and cures to a tough shell. It looks like the more serious product and it is the one most often sold as a dealer add-on. The problem is the seams. A car body is not a smooth surface; it is panels crimped and welded together, and the coating bridges those seams rather than penetrating them. When the shell eventually cracks — from stone chips, flex and age — salt water wicks in, and now it is trapped against bare steel in a place that cannot dry out and cannot be washed.

A lanolin-based product never cures. It stays soft and slightly oily, which means it keeps migrating into seams and crevices over time rather than sealing over them. Nothing gets trapped, because nothing is sealed. The trade is that it attracts dust, it can drip in hot weather, and it needs reapplying annually.

The three, on what they publish

One published durability claim between the three of them, and no coverage area at all. In a category where you are buying an invisible benefit that pays off in five years, a maker willing to print a number is worth a few dollars.

Who each one is for

• Fluid Film aerosol — nearly everyone, starting this month. Ten dollars, no equipment, and the established name in DIY rustproofing. Buy two or three cans. Skip it if you want a stated reapplication interval
• B’Laster Surface Shield — if you would rather do this every other year. The only published durability claim here, at up to two years. Skip it if you are treating a whole underbody, where the cost per ounce starts to matter
• Fluid Film gallon — only if you already own an undercoating gun. The right quantity for a full underbody and far cheaper per ounce, but the gun and compressor are a separate budget. Skip it if this is a one-time job

Doing it, in the right order

1. Do it before the first salt, not after. October is the month. Coating over salt that is already on the metal seals the salt in
2. Wash the underbody first and let it dry completely. A pressure washer or a coin-op underbody rinse is enough; the product needs clean, dry metal
3. Get the car up safely. Ramps or jack stands on level ground, never a jack alone — best jack stands and wheel chocks
4. Target the places that rot first: wheel arch lips, rocker panels and their drain holes, subframe seams, the leading edge of the floor pan, suspension mounting points and the bottoms of the doors
5. Use the extension wand into cavities. Rust starts inside box sections, where you cannot see it, and a straight spray at a flat surface misses the places that matter
6. Keep it off the brakes and the exhaust. Mask discs, pads and calipers, and stay off a hot exhaust — this is an oil-based product

The free half of the job

Coating is the thing you buy. Rinsing is the thing that actually determines whether your car rots, and it costs nothing.

Salt corrodes while it is wet and in contact with metal. A car that gets its underbody rinsed every week or two through winter will outlast an identical car that never does, coated or not. Most touchless washes offer an underbody rinse for a dollar or two — take it every time, and take it on the warmer days when the rinse water will not freeze in the seams.

If you wash at home in winter, a rinseless approach avoids a frozen driveway and a frozen hose — best waterless and rinseless car wash, though neither substitutes for a real underbody flush.

Salt attacks the paint as well

The underbody is where rust kills a car, but the visible damage starts above it: salt spray dulls paint, etches into stone chips and works under the edges of them.

A sealant applied in autumn gives the paint a sacrificial layer through the salt months — best car wax and paint sealants. And any chip that has reached bare metal should be dealt with before winter rather than after, because that is where rust will start on the bodywork.

Door edges take the worst of it from grit and salt thrown up at the shut line — best door edge guards. The rest of the pre-winter list is in best car accessories for winter.