Water-based tile grout used in bathroom areas like showers is designed to harden and become impenetrable. It will stand up to scrubbing, cleaning solutions and warm water and not soften. Over time, the grout stains from mildew and even starts to crack. When it does, you need to soften the tile grout and remove it so you can install new grout.
Set your heat gun at 300 degrees Fahrenheit or less. If you use a blow dryer, place it on the lowest setting. This is particularly important if your shower is lined with plastic tiles instead of ceramic.
Hold the heat gun away from the grout approximately 6 to 8 inches to prevent burning or liquefying the grout.
Keep moving the heat gun around the grout line as you heat the surface. Follow behind with your grout removal tool and remove the grout as you heat it.