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How Road Salt Can Damage Your Classic’s Undercarriage

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Every winter road salt emerges as the unsung hero of road safety, diligently battling icy roads to keep us moving. Yet, beneath this seemingly benevolent effort lies a hidden danger that classic car owners often overlook – the corrosive impact of road salt on their cherished vehicles. 

Whilst road salt plays a vital role in ensuring safe winter driving conditions, its interaction with your car’s undercarriage can lead to a hidden costly damage. The very substance employed to thaw the roads and enhance our security can, over time, become an unwitting saboteur, quietly compromising the structural integrity and longevity of your vehicle. 

In this blog, we delve into the dance between road salt, moisture, and metal, unravelling the mechanisms that contribute to undercarriage corrosion. So, fasten your seatbelt and join us on a journey to discover what’s going on beneath our cars.  

Picture of gritter van
Road salt – great for safety, not great for rust

What Is Road Salt And Why Is It Used? 

Road salt, also known as halite, is a naturally occurring mineral. It’s found in several areas of the UK, most famously around Winsford in Cheshire. Halite, just like household table salt, is primarily composed of sodium chloride. Table salt is purified to meet strict standards for human consumption, whilst road salt is typically sourced directly from mines. Impurities like calcium, magnesium and potassium give road salt its distinctive grey or brownish appearance. 

Road salt is applied to roads because it lowers the freezing point of water. Fresh water freezes at 0 degrees Celsius, but a 20% saltwater solution doesn’t freeze until it hits -16. So, when snow melts and mixes with road salt, it takes longer for the resulting solution to solidify into ice. Road salt is normally applied wet. Wet salt clings to the road better (meaning less is needed) and prewetting the salt helps to start the melting process.  

When salt dissolves, it separates into sodium ions and chloride ions. It’s these ions that help separate the water molecules and hinder ice formation. Consequently, when road salt works it creates a considerable number of free-floating ions. So, when you drive on a treated road your tyres spray up an ion rich brine. There’s an awful lot of this brine, because approximately two million tonnes of rock salt are sprayed across our roads every year.  

Electrolyte solution
Sodium chloride, commonly known as salt, a compound with the chemical formula NaCl

How Does Road Salt Cause Damage? 

Most cars are constructed using steel, a material primarily composed of iron, with a tiny amount of carbon. Iron corrodes due to its inherent tendency to return to its natural state, what scientist call the ‘thermodynamically favoured state.’ Nearly all metals tend to combine with oxygen to form metal oxides. When found in the ground, we call these metal oxides ores. In very dry conditions iron rusts slowly, but in the presence of water iron atoms readily combine with oxygen to form oxide. 

Road salt is very damaging because steel components rust quicker when exposed to water rich in electrolytes. Electrolytes are chemicals that enable the conduction of electricity when dissolved in water. Steel rusts approximately five times faster in saltwater than freshwater. Standing water can also mix with road surface contaminants to form damaging acid, which means when you drive on a treated road, your vehicle is drenched in brine and acids.  

Some people believe road salt is bad for the environment, but the Environment Agency don’t believe this. They claim the rapid dilution of road salt mitigates any potentially negative effects on wildlife. Studies conducted in countries that apply road salt for several months every year have confirmed road salt can harm vegetation and wildlife. US highways studies have proven that salt does accumulate in wetland areas. 

Rusty car floor
Numerous rust patch repairs on a mini

Types Of Damage Caused By Road Salt 

The words rust and corrode are often interchanged, but iron rusts and other metals corrode. The issue with steel is that unlike corrosion, rust does not form a protective barrier and therefore continues to erode. Clearly, a flaky red steel part looks and feels vastly different to a corroded copper brake pipe or an aluminium part that’s developed a powdery finish.  

Road salt can damage every part of your car, but floor, chassis, suspension, brake, and exhaust components tend to suffer the worst. This is mostly to do with where the brine gets sprayed but also because of the natural tendency to only wash what can be seen. For this reason, rust tend to start in the dark under your car and works its way up. It’s not unusual for a car that looks to be in good condition to fail an MOT due to rot below the floor line.  

Every classic car owner knows the tell-tale signs of rust formation – bubbling, rust spots, discolouration, flaking, pin holes and so on. These are obvious on body panels, but rust under wheel arches and beneath the floor is less obvious. This means getting down to floor level and using a torch to look for anything that may be of concern. If you take your car to a garage with a ramp, they may let you have a look under the car whilst its off the ground.  

Side of rusty van
Rust often starts low down near wheel arches

Preventive Measures – Washing 

The key word to remember here is ‘dilution.’ If your car is sprayed in brine the best thing you can do is rinse it thoroughly using plenty of freshwater. It’s generally agreed that you should wash your car at least every other week in the winter, but if you have classic car, it’s a particularly clever idea to rinse your car every time you drive on a road that been treated with road salt. 

If you’ve never cleaned the underside of your car you might need to use a cleaner and a brush to remove any crud that’s built up, but it’s far better to make chassis cleaning part of your regular cleaning regime than seeing it as something you do once a year when the weather improves. Regular rinsing also reduces the amount muck that clings to the bottom of your car.  

Don’t be tempted to blast your classic car with a pressure washer. Owners of new cars might get away with this, but pressure washers are generally a bad idea when it comes to classic cars because they’re too powerful. They shift dirt, but they often damage the very barriers designed to help prevent rust. Jet washing can also blast water into cavities and electrical components.  

Washing and rinsing the underside of your car often over the winter months may appear a bit of a faff, but using wash wands or cleaning trolleys that simply connect to a standard hose makes the job quick and easy. They don’t use a huge amount of water and no chemicals are needed. Adding five to ten minutes to a body wash will pay dividends over the years. Kits available here

Preventive Measures – Barriers 

The key to slowing down rust is shielding steel from its surrounding atmosphere. The first and often only line of defence is paint. Paint can certainly be an effective barrier, but stone chips can expose bare metal and a rust can work its way under your paint. If you’re going to rely on paint alone, it’s important to check it regularly and attend to any breaches before rot sets in.  

The next thing to consider is what might be called Paint Plus – products designed to either replace traditional paint or add another layer of protection. For example, Raptor paint is being used for many restorations, being described as ‘almost impenetrable.’ There are many other types of modern protective paints and of course there’s also powder coating, where an electric charge is used to fuse a dry powder to the steel before being cured in an oven.  

It’s impossible to talk about classic cars without mentioning underseal. Generally speaking, old fashioned bitumen type underseals have fallen out of favour. Underseal is accused of being messy, difficult to get remove, hiding problems and worse still actually trapping moisture. That said, you can certainly find classic cars undersealed from day one that remain very solid. 

The next thing to consider is waxes and sealants. Probably the best know of these is Hammerite Waxoyl, which is often used in conjunction with their underseal product Bodyseal. Other products to consider include Bilt Hamber Dynax UC, Liqui Moly and Dinitrol. An interesting product is Lanoguard, which contains Lanolin, a natural substance derived from sheep’s wool and other ingredients such as beeswax and mineral oil.   

It’s also worth considering how your car is stored. Garages and covers can stop a car getting wet, but a badly ventilated garage or cover can create the humid condition rust loves. If budget is not too much of a consideration for you it’s worth looking at a Carcoon, which has a protective airflow system. We recommend you rinse and leave your car to dry for a while before you put it away.  

Painting to protect against rust
Paint protect steel as long as its not damaged

The costs of rust 

It’s extremely hard to quantify the damage rust causes in monetary terms, but it’s clearly a massive number. Between January 2019 and June 2021 almost 1 in 5 of the 120k classics over 40 years subjected to an MOT failed. It’s now known how many failed due to rust, but every MOT tester has horror stories of holes, weak structures, and catastrophic failure of parts due to rust.  

Classic cars tend to be taken to restorers when rust has taken hold and the restoration market is a £5.5bn industry in the UK. Panels for most classic car aren’t outrageously expensive, but replacing panels is labour intensive and labour isn’t cheap. A simple patch to get a car through an MOT costs £60-80. New floor panels for a Morris Minor might costs around £200 but fitting them can cost in the region of £1,000 to £1,200. 

The TV programme Bangers and Cash – Restoring Classics featured the restoration of a 1976 Saab. The car sold for just over £20k restored, which only covered half the cost of the restoration. With restorations often costing more than the value of the finished car many owners can only justify work on an emotionally level rather than financially one.  

Classic car owners who are good with their hands and like a challenge have for many years undertaken DIY body repairs, but now many owners have no choice but to teach themselves to weld because they simply can’t afford to pay someone else to do the work. As we’ve seen rust also attacks parts and these are often also expensive, for example, replacing front and rear subframes in a mini cost the best part of £1,000 for genuine parts.  

Front of Ford car
Body repair is always expensive on classic cars


Pretty much every classic car owner is aware of rust damage and take some precautions against it. Rust is generally seen as something to loath and to battle. However, out of sight is out of mind, so body work and chrome are often given priority over what’s going on in the dark underneath. The damage road salt can cause really isn’t given sufficient attention.  

Around half a million cars over 30 years old registered with DVLC and it’s been estimated roughly 70% of those cars are used year-round. However, classic cars could certainly be used more. Reseach suggests the average classic car owners only uses their car 18 times a year, clocking up just 1,124 miles. Taking cars off the road for the winters is understandable, but it’s also a shame not to see them being used over the winter months.  

Rust champions ClasX – “As Benjamin Franklin told threatened Philadelphians – An ounce of prevention is worth a pound of cure. We want classis car owners to get into the habit of always washing road brine off their car before they put it away. This will only happen if this isn’t seen a being too much of a faff and our products are making this task quick and easy.” 

Long Wash Wand picture
A long undercarriage wash wand, only from the ClasX website

Additional Resources 

Science – Why Does Metal Rust? – Reactions Q&A – YouTube 

Deep Dive – Rust and Corrosion: A 10 minute guide – YouTube 

Source info – One-in-five classic cars fail the MOT test, despite not legally requiring one – The Garage and MOT Magazine 

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