What Causes Water Spots on Cars? Here’s The Real Answer.

If you’ve ever owned a car, you’ve seen a water spot. Small, white-ish stains on your otherwise immaculate finish, they leave many asking just what causes water spots on cars. Simply put, they come from beads of water resting on your paint, which dry and leave behind a residue. That’s why it’s so important to have a ceramic coating that will either let those beads roll away or sheet them off immediately before they have a chance to dry.

So that’s what causes water spots on cars—static beads! But what is it exactly about those beads that forms the water spot? What’s happening inside a bead to cause such a nasty stain? Well, as with anything, there’s a simple answer and a not-so-simple answer. We’ll give you both:

The simple answer

Water can be divided into two categories: hard and soft. No, we’re not referring to alcohol content—we’re talking content of the mineral variety. Water with a high amounts of magnesium or calcium is said to be “hard”, while water with low amounts of those minerals is called “soft”.

Mineral content of water is largely determined by the geology of the water source’s surrounding area, so if you’re somewhere with a lot of limestone, chances are your tap water are very hard. That being said, though, just because your water is soft doesn’t mean there’s no mineral content—soft water actually contains high levels of sodium.

Getting back to water spotting: When a bead with mineral content evaporates, it leaves all of that mineral content behind. After all, solids can’t evaporate like liquids do; that’s just physics. That remnant forms a residue, which while at first is easy to remove, will soon become a stubborn stain that may even etch into your finish.

The less-simple answer

Imagine a bead sitting on paint. Because of the bead’s spherical shape, less water is present on the edges of the bead. Evaporation rate increases where less water is present, so the outer contact line of the bead evaporates first. That’s where things start to get interesting.

Because the bead’s contact line is static, as water disappears, water from the rest of the bead rushes to the edge to take its place and maintain the contact angle. That rush of water takes mineral content along with it, causing it to accumulate at the edges. Once the water is gone, UV radiation causes that remaining residue to bond chemically with the paint. With this, the process is complete—you’ve got a water spot.

Oddly enough, this exact same process is what causes a coffee stain to occur. Just sub out mineral content for coffee bean particles, in this case.

What to do about water spots

Now that you know what a water spot is and how one forms, how exactly do you prevent them? And if you can’t prevent them, how do you remove them? We’ll be answering those questions over the next few weeks—stay tuned.

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