regrout over existing grout?

The tile contractor didn't do a good job while setting the tiles in my kitchen; in some locations the grout doesn't fill the joints properly - it's too low. (I know it should be slightly lower than the tiles, but in these cases it's too much).
I would like to regrout the floor to fix the problem. Is it OK to do this over the existing grout (was set couple of months ago)? Is it going to stick to it? These are polished porcelain tiles, and grout is less than 1/8" thick.
Thanks, Matt
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In my (limited) experience with grouting, you shouldn't put new grout over old grout completely. I believe I read on one of the grout boxes that if you're re-grouting, atleast 3 mm of grout need to be cut out first. To cut out some of the old grout, you can use a rotary tool or get a grout saw at the hardware store.
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A utility knife with razor blade is good for removing old grout.

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Never add new grout over old grout. It will not stick.
You must remove some of the old grout, the more the better. Try and remove atleast 1/8" to ensure a good sized crevice to fill.
snipped-for-privacy@hotmail.com wrote:

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snipped-for-privacy@hotmail.com wrote:

Do you mean the grout is 1/8" _wide_? And how deep are the bad areas? How much of the job (percentage wise) is not filled properly? 10%? 50%? Are there parts of the job where the grout has filled the joints completely, or is it all basically 'washed out'?
thetiler
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