Road salt and rust: how to protect your car this winter
Road salt keeps traffic moving when temperatures drop, but it is tough on cars. Salty water and slush get sprayed into every gap underneath, where they speed up rust on the underbody, brake lines, suspension and exhaust.
The good news is that a bit of regular washing and some simple protection can keep it in check. This guide explains which parts are most at risk, how to clean your car properly in winter and when corrosion becomes an MOT problem.
Last updated 9 November 2026.
Key takeaways
- Salt speeds up rust by helping water react with bare metal. Damp, mild winter weather is often worse than a hard frost.
- The underside, wheel arches, sills, brake pipes and exhaust are most at risk.
- Rinse the underside and wheel arches every couple of weeks in a gritting spell, and after snow.
- Wax before winter, and consider an underbody protection treatment for older cars.
- Serious corrosion on structural parts, or near brake, steering and suspension mounts, can fail an MOT.
In this guide
- Why salt causes rust
- Parts most at risk
- How to wash salt off your car in winter
- How to protect your car before winter
- When does rust become an MOT fail?
- Check for rust when buying a used car
- Frequently asked questions
- Sources and further reading
Why salt causes rust
Rust forms when iron or steel reacts with water and oxygen. Salt dissolved in water makes it conduct electricity better, which speeds up the reaction. Every time you drive on a gritted road, a salty spray coats the underside of the car.
Modern cars are much better protected than older ones, with galvanised panels and factory coatings. But chips from stones, scrapes on speed bumps and worn coatings can expose bare metal, and once rust starts, it spreads underneath the paint.
Parts most at risk
- Wheel arches and sills, which catch spray and trap mud.
- Underbody and subframes, which hold the suspension.
- Brake pipes and brake discs. Corroded brake pipes are a common MOT fail on older cars.
- Exhaust system, especially the back box.
- Suspension springs and mountings. Corroded springs can snap.
- Door bottoms and boot lip, where water collects.
- Stone chips on the bonnet and front panels.
How to wash salt off your car in winter
- Wash regularly during gritting spells, ideally every week or two, and after snow.
- Focus on the underside and wheel arches. Use a hose or pressure washer to rinse them thoroughly. An under-car spray attachment helps.
- Choose a car wash with underbody spray if you cannot wash at home.
- Rinse first, then wash with car shampoo and a mitt, so you do not rub salt and grit into the paint.
- Dry the car, especially door shuts and seals, so water does not freeze.
- Wash on milder days where possible, so water does not freeze on the car or the ground.
How to protect your car before winter
- Wax or seal the paint in autumn. It creates a barrier against salt.
- Touch up stone chips before the metal underneath starts to rust.
- Check drain holes in the doors and sills are not blocked, so water can escape.
- Consider underbody protection. Garages and specialists can apply wax or anti-corrosion treatments to the underside and cavities. This is most worthwhile on older cars, classics and cars you plan to keep for a long time.
- Use mud flaps to reduce spray on the sills.
When does rust become an MOT fail?
The MOT checks for corrosion that affects the car’s structure or safety. You can expect a fail if there is:
- excessive corrosion on structural parts, or within 30cm of prescribed areas such as brake, steering, suspension or seat belt mountings
- corroded brake pipes or hoses
- holes in the exhaust that cause leaks
- corroded suspension springs that are cracked or broken
Surface rust on body panels is usually an advisory rather than a fail, but it is a sign to act before it spreads. Our guide to common MOT failures covers the other big ones.
Check for rust when buying a used car
If you are buying a used car in winter or spring, look carefully at the sills, wheel arches, underneath and brake pipes. Check the MOT history online for corrosion advisories. See our used car inspection checklist.
Frequently asked questions
Is it bad to wash my car when it is freezing?
Water can freeze on the paint, locks and seals. Wash on a milder day if you can, dry the car well and avoid locking it while seals are wet.
Does rust affect my insurance?
Rust is wear and tear, which insurance does not cover. Severe corrosion can also make a car unroadworthy, which could affect a claim.
Do electric cars rust?
Yes. Electric cars have fewer exhaust and engine parts to worry about, but their bodywork, subframes, suspension and brakes are still exposed to salt.