replacing old tile grout

The tile grout is very old some of it is cracking and falling out. I bought a tool to scratch out the grout before I put in the new grout. Or Should I just put the new grout over the old grout? I forgot to mention its in a bathtub and shower area.
thanks mike
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The space for the grout has to be large enough for the grout. If you put it on over the old grout, it will flake off over time. Scrape out all the loose grout with a razor knife, then scrape the rest with your carbide tool. Rinse to remove any loose grout that it still in the cracks and you should be set. I usually wash down the grout that there with a bleach solution to clean the remaining grout to see if there's other damage. Rinse well so the bleach doesn't remain on the tile or grout.
I usually use caulk where the tile meets the tub as this cracks easily; use one that's mold resistant.
charles
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