Which statement about installing mirrors is NOT correct?

The statement indicating that mastic should cover 90% of the back of the mirror is incorrect. In mirror installation, it is generally recommended to use mastic in a spot-application manner, covering only a small percentage of the back surface—typically around 20-25%. This method allows for sufficient adhesion while ensuring that any expansion or contraction of the mirror does not lead to cracking or damage. Covering such a large portion of the mirror could also lead to entrapment of moisture behind the mirror, which can promote mold growth or damage the backing. Thus, the proper approach is to use spot applications that effectively secure the mirror while allowing for some air circulation and flexibility.