A mirror backing paint is used to prevent a silver finish from corrosion or attack by solvent(s), such as when using a mirror mastic. While some mirrors have a copper layer as a direct backing agent (in case moisture or air leaks through), most mirrors have a good backing paint which either binds directly to the silver or seals an existing copper layer over the silver to protect it. It is highly recommended to use a mirror backing paint to protect your silver whenever possible, unless of course if you are going for an antique or tarnished silver look. If you are looking for a one-way mirror look, we suggest using a clear backing like our Copper-Tite Vehicle #3. If you instead want to give the mirror a dark backdrop or contrast, we suggest using either our #390 or #175 backing paint.