I am assuming that the tile is good and your leaving it in place and just want to remove and add new grout? Right?????
300 sqft, is allot of area for a dremel, you would probably run it dry on this size project.
I would consider a Roto-Zip with the grout blade. It works like a router and you can set the blade or bit depth and eat it out.
Also, you might consider getting a router with a 1/2 inch cheap bit and try it. It would surely be faster, but the dust will be something to recon with.
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