After my failed attempt at cutting down my first builder’s mirror, I got a hold of 2 more and tried again (after watching some videos on the subject and buying a better scoring tool.) I successfully cut them both. One I cut into 7 pieces so I am sure it wasn’t just dumb luck.
Amazon affiliate links: Fletcher Terry 01-711 Fluid Glass Cutter, Fletcher Terry 09-541 Fluid Glass Cutter
2. Apparently you need oil! Either on the mirror or on the scoring wheel or both. I think you can use any kind of oil, sewing machine, cooking…not sure. I just bought the oil that had the same brand name and was meant to be used with my new cutter. (Because I wasn’t sure this would work and I wanted to do everything I could to make a good effort.)
What not to do: I tried holding a piece of melamine as a straight edge which just slid around on top of the glass. *You can use anything with a straight edge, just clamp it down before scoring.
So there you go. It is possible to cut a builder’s mirror.
There was a little chipping right along the edge but nothing a 1/4″ lip on a frame won’t hide so I’m totally happy. It was actually quite fun!