You can use it to clean build up. Lower the water table and pour a 2 liter coke in. Let sit. That's the basic concept.
Put Coke in toilet last thing at night and leave till morning. Toilet should be cleaner and any build up be dispersed.
Better yet use this formula- It cleans everything in the bathroom
Heat up 1 cup of white vinegar, mix in about 1/4 cup of Dawn Premium. Not the dollar store kind, because it is too thin. When mixed, pour into a spray bottle and fill with water. Shake. Spray down the grout and let it sit overnight. In the morning rinse. If you have stubborn stains, spray again. Use this mixture a few times a week. It will keep your grout and tiles clean. Also works wonders in the toilet bowl to keep hard water stains at bay. Oh you only have to heat the vinegar one time when you add the Dawn.