A. Michael Byrne responds: If the reason you’re removing the grout is that it’s old and crumbly, try a utility knife first. If the grout is soft enough, this will work, though you may go through a lot of blades. For wide grout lines — 1/4 inch or more — you might try this technique: With a steady hand, use a dry-cutting diamond blade in an angle grinder or circular saw and cut out the middle of the joint. Then cut away the rest with a utility knife (set the blade only as deep as the tile).
For really persistent grout and narrow joints, there’s no easy answer: It’s a tedious and time-consuming process that’s unforgiving of mistakes — you stand a good chance of slipping and scoring the tile.
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