ANSWER: Any GF Water-Based Finish can be removed from a mirror using 0000 steel wool and Windex. However, you can prevent a mistake from happening by applying Frisket first. More details below:
Prevention of Finish Spills: Apply Frisket to your mirror BEFORE applying finish to the frame. Frisket is a clear finish that brushes on, dries and peels off when you're done painting
Removal of Finish: Wet the steel wool with Windex and buff the glass. (The steel wool won't harm the mirror, but make sure that you don't apply so much liquid that it seeps into the backing.) Wipe the mirror down with a clean/dry paper towel.
*Windex Warning: Make sure you do not get Windex on water based finishes. The ammonia in Windex will cause a cloudy appearance to the finish.