Underbody Coating: Essential or a Waste of Money?

Plenty of owners have been offered an underbody coating at a dealer or a detailing shop. But what does it actually do, and is it worth doing? A few facts first.

Take a look underneath a car that has run for years on rough roads and the picture is rarely pretty: rusted areas, stone chips, and rain doing its work. On a new car the symptoms are less obvious, but without protection they appear within a year or two.

As one long-experienced driver puts it: roads are not all motorway smooth. On rough, muddy tracks you hear the underbody strike something, and the car takes a constant series of impacts. Over time rust works deeper into the metal until the floor pan perforates and the car is finished.

In practice a coating is necessary, and it is not where to save money. If the chassis rails rust through, the safety consequences are serious. At speed, stones thrown up do real damage: a 10 gram stone at 80 km/h strikes with a force about 30,000 times its own weight, enough to break through paint films thinner than 30 microns. Once the film is broken, rust starts at the chip and spreads slowly through the steel.

So yes: a coating earns its place, particularly for cars that live outdoors or on rough roads.