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How Road Salt and Deicing Chemicals Damage a Car’s Finish

Road chemicals used to pretreat for winter weather are damaging to automobile finishes.

Salt and Rust

We are all quite aware that road salt causes vehicles to rust even if we aren’t keenly attuned to the exact chemical process. Without bringing up too much of the high school chemistry buried deep in our pasts… rust, or iron oxide, is the result of a reaction between iron in the metal and oxygen in the air. Water by itself doesn’t do much to further the reaction between iron and oxygen, but water containing lots of free floating ions (like sodium and chloride ions from dissolved salt) kick the electrochemical process into high gear.

Other road salt deicers, like calcium chloride and magnesium chloride, involve more complex molecules that distribute more ions when they dissolve in water. When dissolved, these molecules can prove even more damaging to a vehicle’s sheet metal than sodium chloride.

Once the rust process begins, it works quickly. Iron oxide molecules take up more space than iron atoms, so their expansion tends to break metal apart. Carbon dioxide in the air and water combines with the iron to create iron hydroxide, another form of rust that easily separates from the base metal. The rust process doesn’t stop unless all the corrosion is removed and the base metal is protected by some form of barrier – like a high quality primer, paint and clearcoat combination.

To make matters worse, as temperature increases in the Spring the rust reaction speeds up. All chemical reactions are temperature dependent. The warmer it gets, the faster the molecules move around – bouncing together and reacting at a higher rate. Plus, the warmer temperatures mean more moisture in the air, adding to the equation.

Clearcoat Penetration

So it’s easy to see how rust can form on bare metal. It makes sense that we should take care to rinse exposed metal (such as that in wheel wells, suspension, and undercarriage components) as much as possible throughout the winter. But how do road deicing chemicals harm painted surfaces?

If we were to look at a painted surface under a very highly powered microscope, we would see that it isn’t a solid, impenetrable surface – it actually has pores. Like most matter, sheet metal expands and contracts depending on the temperature. If paint were not able to expand and contract at the same rate as the metal to which it bonds, it would crack on a particularly hot or cold day.

Now, the pores that we are talking about are too small to allow water and salt ions to penetrate onto the metal below. However, over time the ions set free in the salt dissolving process actually create a mild acid, which attacks the clearcoat surface. Plenty of other contaminants and impurities get into the salt slurry to further acidify it. This acid will slowly break down the clearcoat and cause it to discolor, fade, and eventually fail.

Damaging Car Washes

Unless we are lucky enough to store our cars for the winter in a temperature controlled environment, it is inevitable that we’re going to end up with a coating of salt all over them. At this point the goal should be to get the salt off of the car as quickly as possible – especially before the temperature starts to warm up.

Automated car washes are convenient, but most have flaws that can actually do more harm than good. The cloth brushes of a regular automated car wash become saturated with the dirt and grime of the previous cars and then act like sandpaper – gouging and scraping the clearcoat on your car. Scratches and swirl marks in the clearcoat just help the deterioration process along, providing crevices for contaminants and acids to embed further into the surface.

Brushless car wash you say? Not so fast. These aren’t a whole lot better. Most use high pressure water and harsh chemicals to make up for the lack of any physical brushing. The problem is that most touchless washes use reclaimed water – water full of salt, road grime, and other chemicals. The high pressure water acts as a sandblaster, pelting your clearcoat with dirty water. Touchless car washes also use harsh chemicals (usually an acidic wash followed by a basic wash) to lift and rinse dirt and grime away. These harsh chemicals are also tough on clearcoat finishes.

You’ll still need to wash your car, at least monthly, through the winter. So choose your car wash carefully. Avoid brushes, stick to newer brushless washes, or wash the car yourself at home or at a self service bay. Better yet, have a professional detailing company wash the car for you!

The Best Offense is a Good Defense

You’ve heard the saying, but how does it apply? The best way to prevent clearcoat damage is to have a layer of protection between the clearcoat and the harsh road chemicals. Having a high quality wax applied professionally at the beginning of the winter is a great first step. A wax will act as a physical barrier, filling in the small scratches and swirl marks, and coat the entire surface. Wax will last between one and three months but is quickly removed by a car wash or two. A better option would be to have a sealant applied. A paint sealant is a synthetic wax, a manufactured product that will act like a wax but last longer – up to 6 months. The best option is to consider having a permanent coating applied. A permanent ceramic coating, such as Opti-Coat PRO chemically bonds to the clearcoat, creating a highly durable, chemically resistant layer over the clearcoat – protecting it for years to come. Salt, road grime, and other harsh chemicals will not be able to penetrate the ceramic coating and will rinse away with ease.       

Protecting your vehicle before winter sets in and maintaining a regimen of gently rinsing salt away will help prevent rust and clearcoat failure and keep your car looking great.

3 Comments
  1. I never knew that rust could be such an issue for even newer cars with bad paint. I know the clear coat on my car is starting to peel in certain places and I thought it would just be an aesthetic issue. I’ll have to get it painted quick to make sure any already existing rust does not spread. Thanks for the awesome info!

    • Thanks for checking in David! Rust is always an option – even on newer cars. Failing clearcoat is a sign of trouble ahead and should be repaired quickly. Make sure to use a reputable shop that will take the time to prepare the surface correctly and use high quality materials.

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