Take Cover!

American households spend hundreds of dollars a year on auto maintenance—fluid changes, new tires, tune-ups and body work—in order to make sure that their cars are ready for anything the road has to throw at them.

But to keep your car looking its best, you also need to be ready for aerial assaults. Because the truth is that two airborne menaces—bugs and bird droppings—can seriously degrade your car’s appearance, and can, if not addressed, damage your paint and leave your car’s surfaces vulnerable to weather, pollution, and other corrosive forces.

Bugs and droppings may not seem like much of a threat. But when you consider how little paint there actually is on your car’s surfaces, it’s not that big a surprise that they can cause some serious damage. Manufacturers generally apply car finish in three layers: a layer of primer, a layer of tinted paint that gives the car its color, and a layer of untinted paint (commonly called clear coat) that protects the colored layer from the elements and from UV rays. But the first two layers are usually slightly thinner than a human hair. And while the clear coat tends to be thicker—two to three times thicker than either the primer or the paint—it’s still incredibly thin. So thin, in fact, that bug splatter and bird droppings present a real challenge to the clear coat’s ability to protect the all-important layer of paint that gives your car its color. But each of them do it in their own way.

To us, houseflies, lovebugs, and other flying insects are tiny. But on the scale of the thickness of a layer of paint, even a humble mosquito is thirty times thicker, and comes in like a 747 landing on a kiddie pool. Still, flying insects are pretty fragile, so they wouldn’t be much of a problem if it weren’t for their payload—the distant cousin to blood that sloshes around in the insect circulatory system.

This blood-like substance is acidic, and it only becomes more so as it’s exposed to air and sunlight. The residue of a lovebug, for example—the firefly-sized insect that throngs roads in Texas and other southern states in spring and summer—starts out only slightly acidic, but over 24 hours it can become more and more concentrated, until that blob of insect goop gets as caustic as tomato juice.

It may be hard to believe that nature can produce an acid that’s strong enough to etch your paint. But consider acid rain: at the height of the acid rain problem in the early ‘90s, it was blamed for routinely marring car finishes, especially as the rainwater on a car evaporated in sunlight and the acid it contained became more concentrated. These days acid rain is less of a problem, but all acids—even lovebug blood—that come in contact with your car can have the same effect, setting up a reaction with your car’s finish that can leave you with pitted clear coat and discolored paint.

That’s because acid’s effect on car paint is simply a matter of chemistry. Ordinary water molecules are famously made up of H2O—two atoms of hydrogen and one atom of oxygen. But those atoms are constantly breaking up and joining back together, so any water you encounter will actually be a bunch of H2O plus a bunch of hydrogen ions which have a positive charge, and hydroxide ions, which have a negative charge.

Because these ions are roughly equal in number, they balance out, and can react with each other instead of with whatever else they encounter, making water one of the least caustic substances in nature. But if you add an acid to water, then suddenly there are far more hydrogen ions floating around, ready to react with whatever they come in contact with.

pH is a shorthand way of saying exactly how many hydrogen ions there are relative to hydroxide ions. Water has a pH of 7, which means that it has about as many hydrogen ions as hydroxide ions. But as you go lower on the pH scale, hydrogen ions become more concentrated. This means that a pH of 6 is ten times more caustic than water. A pH of 5 is 100 times more caustic. a pH of 4—in the neighborhood of day-old bug splatter—is 1000 times more caustic than water. And once that caustic substance comes in contact with your car, the surplus hydrogen ions begin to react with the polymers in the paint, eating into the stuff that’s supposed to protect your car, weakening it and etching it away.

What’s worse, the UV protection mixed into the clear coat paint tends to be concentrated close to the top of that layer, meaning that bug splatter can compromise the clear coat’s ability to shield your paint from harmful UV rays. At the same time, weather and all the corrosive substances that abound on the road are also doing their best to degrade your paint job, making the clear coat more and more vulnerable to the damage that bugs can cause.

Finally, there’s one more thing to remember about the clear coat: it’s supposed to be clear. One of its main functions is to add gloss and depth to your car’s color. When it gets marred, this changes the way light interacts with it, creating the appearance of damage to the paint even if the damage is only to the surface of the clear coat.

Bird droppings present another significant threat to your car’s paint job. But that’s not because of chemistry: As the British car-care company Autoglym discovered after studying the problem extensively, bird droppings actually warp the surface of your clear coat.

The urethane paints that are most often used for cars and trucks are used for that purpose because they resist heat especially well. But that doesn’t mean that heat has no effect on your car’s finish. Direct sunlight and hot weather (as well as the heat generated by the car itself) can make auto paint soften and expand.

If there’s nothing on top of the paint, that’s not a problem, but the same heat that softens paint makes bird droppings—a mixture of urine and solid waste—naturally dry out and harden. When night comes, or conditions otherwise change, the cooling paint contracts around the hardened dropping, leaving an imprint in the clear coat that distorts the color and texture of the underlying finish. In other words, bird droppings may not damage your paint directly, but the effect they have can still be unattractive and permanent.

While the Autoglym studies established that the urine in bird droppings wasn’t acidic enough to eat into the surface of the paint, that doesn’t mean that you don’t have to worry about chemistry. Because when your car’s paint expands, it also becomes more porous, which allows the uric acid in bird droppings to work its way into your car’s paint job and react with it on the inside, making it harder to prevent further damage.