A car wash might be needed for many reasons: it’s just been through a wicked demolition derby, or even just a bad winter. But car washes do more than spruce up a dirty car. They can also involve detailing options, such as a coat of wax, which might seem like an aesthetic luxury. Besides making a car look extra shiny, what purpose does wax serve? The short answer: it’s the car’s best protection against moisture damage.

How Does Wax Protect Car Surfaces?

You may have noticed that water beads up on the surface of a freshly waxed car. This happens because wax is hydrophobic, meaning that it does its best to avoid absorbing water (and the paint-damaging acids and other contaminants that may come with it).

But why is wax so hydrophobic? Because it’s (quite literally) water’s polar opposite. Water is made up of polar molecules, which are asymmetrically charged particles that are easily attracted to dirt and metal. So when rain falls on an unwaxed car, the water starts to spread out.

On the other hand, wax is nonpolar. If a car has been treated to a coat of wax, its surface becomes much less adhesive and much more like an ice rink covered in banana peels. This leaves the water with nothing to stick to but itself, causing it to scatter into round beads that easily roll right off the surface.

When Do You Need Fresh Wax?

Look at the size of any freshly fallen water beads. If droplets are more than half an inch in diameter, then it's time for a fresh coat. In general, cars benefit from an application of fresh wax every three months, though more frequent waxing is required if cars are regularly driven through harsh or snowy climates, or parked in the first two rows at SeaWorld.