That is a very common question. The material is not defective, as many fabricators have claimed. That is the nature of the material. Acrylic is a hydroscopic material, that is to say, it has an affinity for water and it will absorb moisture into the microscopic pores in its surface. Normally, in the cast of clear acrylic without the mirror backing, it absorbs moisture evenly on the face side and the backside and releases it evenly depending on the relative humidity.

The bowing in the material occurs when you seal the backside with a non-porous coating, which prevents the normal absorption or release of moisture on the coated side. This produces an imbalance within the sheet, which may be exacerbated by conditions of high humidity during the summer months, or near coastal areas where humidity is highest.

For most applications, this characteristic of acrylic mirror is not a problem, but should be considered if flatness is of critical importance in your application. Glass mirror, for all its obvious limitations is still a better choice for flatness. Just don’t drop it!