On Mon, 18 Oct 2010 17:28:16 -0400, "
wrote:

I've replaced grout in a shower and it is a fairly big project,
but well worth it.

One great tool is a grout saw. You only need to take the tile grout
down a little, but get the grime out when using it. Then top with some
new grout and later seal...

http://images.drillspot.com/pimages/2780/278045_300.jpg