Underseal

Underseal (often called undercoating in the U.S.) is a thick, resilient coating applied to concrete, and to the underbody or chassis of an automobile to protect against impact damage from small stones, which would rapidly chip ordinary paint, allowing rusting to begin.

On vehicles

Historically, a bitumen-based compound was used, but after approximately a decade, such coatings become brittle, and water can become trapped between the underseal and body metal, counterproductively creating a more favourable environment for rust than if no underseal were applied at all. Wax-based underseals do not have this disadvantage, but can be eroded and eventually washed if a pressure washer is used to clean the vehicle's underbody. Rubber-based underseals are also sometimes used.

Vehicles for sale in some territories are not undersealed, because the climate is not sufficiently aggressive to warrant it. If such a vehicle is subsequently imported into a country with some combination of high rainfall, cold winters, and salting of roads to prevent ice, application of underseal is often used.

On roads

When concrete on streets, parking lots, or warehouse floors begins to degrade, there are a few methods of repair available. Concrete undersealing is a process by which hot asphalt is pumped underneath the concrete slab so that it flows into hollow and eroded patches. After concrete undersealing, the concrete is more stable and can be overlaid with a thin layer of asphalt to repair cracks, saving the expense and time of replacing the entire surface.