In order for plastic to be reflective, it has to be coated with a layer of metal. That metal layer, however, is so thin that it is extremely vulnerable — and it needs to be protected. Without it, the metal can easily be scratched, chipped or chemically attacked, leading to spots in the mirror that are no longer reflective. The protection comes from a layer of paint called back coating. The type of back coating that mirror manufacturers apply depends on the conditions that the mirror will have to endure. There are two kinds of back coating that offer two different levels of protection.

1. Water Reducible
Water-reducible back coating is only recommended for mirrors that will be used indoors, and that won’t have to withstand weather or harsh treatment. It’s the least weather durable, but when the application is indoors it is perfectly adequate and is the most environmentally friendly option. Water reducible back coating is ideal for convex mirrors that hang in the aisle of a grocery store, for example. These mirrors are indoors, they don’t have to stand up to the elements or temperature swings, and they won’t be exposed to harsh chemicals.

2. Solvent Reducible
Solvent-reducible back coating is the right choice for mirrors that have long-term exposure to weather, such as mirrors on the exterior of trucks or buses, or mirrors used on European roadways. This type of coating is much more durable than water-reducible coatings, providing greater protection against chemicals, abrasion, cold, heat, sunlight and the elements. For example Replex has successfully tested its solvent reducible coating system for up to 2000 hours of salt-fog exposure. This performance exceeds most OEM truck manufacturer’s specifications by two to ten times the specification.

The type of coating that will protect the thin layer of metal on every mirror depends entirely on what that mirror is required to endure. Knowing what your mirror will have to tolerate is the first step in deciding what kind of coating protection it needs.