I have used a roto zip with a tile bit and it didnt last that long and the bits can be expensive, a grout saw for a small area is the best I have found or for a large area I have used a grinder with a four inch diamond blade with a vacuum right down by the blade.
If the area is small it is just as easy to remove the grout by hand with a grout saw.If there is alot I like using a roto-zip equipped with a tile cutting bit. Works like a charm and its fast. Ill be discussing this at our seminar in January and if you twist my arm Ill bring my roto zip with me.
Dr. Fred
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Don Kasch
Floor Restoration Specialists
floorrestorationspecialists.com
801.358.0423