1. Boil 3 tablespoons of white vinegar with 3 tablespoons of water.
2. Dip the rag in the hot vinegar and water mixture, being very careful not to burn yourself.
3. Rub the rag on the painted glass, slowly at first as the heat loosens up the paint and then scrub a bit harder once the paint starts to come up.
Additional Notes: This worked to remove our paint in minutes, but if the paint is very stubborn, you can use a razor blade to scrape off the paint once it has been loosened by the hot vinegar mix. Be sure the surface is wet and scrape very carefully so you don't scratch the glass.