DIY Painted Mugs That Won't Wash AwayThere are a few baking rules which are all printed on the back of the marker for easy reference. 1. Don’t preheat your oven. Put your piece in first then turn on the oven to 375 degrees. 2. Set your timer for 40 minutes AFTER the oven has come to temperature, not when you put the mug in. 3. Don’t take your mug out when the timer goes off. Instead, turn off the oven and open the door. Let the mug cool as the oven cools.