There are two ways you can make the paint more permanent on a glass surface. First, you can leave the paint to air dry for at least 21 days. The second method is oven baking, which is the better option if you want your glass item to last. Place glass items in a cool oven and then set the temperature to 350 degrees Fahrenheit (176.67 degrees Celsius). Allow the glass to heat up with the oven, and then leave to bake for 30 minutes. When you switch the oven off, make sure that you leave the glass items to cool in the oven and wait until it is completely cool before opening the door. Always wait at least 24 to 72 hours after curing before you wash the glass.