Personally I don't do re-grouts because they are always
questionable at best. Grout is made to go in- between the
tiles and lock in underneath them. When you re-grout,
you're only trusting in a thin surface bond. A complete
removal as SQLit mentions would be acceptable because
the grout could get deeply into the joint to grab.
I don't know why the tilelab stuff didn't work. For floors
I use the darker, expensive ($15 pint) bottle of Tilelab
and it seals polymer grout in one heavy coat. You can
dump water on it when dry and it beads and will not soak
into (or darken when wet) the grout.
I've also never sealed shower tile. For one, it's hard to get
the sealer to lay on it long enough to dry, because it's a
vertical surface. Secondly, I have a personal problem with
all that moisture and running water getting into the grout,
yet not being able to dry out because the sealer is holding
the moisture in. I know sealers are supposed to be
"breathable" but by the way they make water bead on the
surface, I question how much moisture can get out of
I don't use darker colored grout in showers for the reason
you mentioned. Pure white (sanded or unsanded) will not
darken when wet and looks great with any tile in a shower.
At the most I use a light-to-medium color that won't darken
so bad, and warn customers in advance that it WILL darken.
I like to just do a good job grouting, and leave it unsealed
in wet areas. Moisture and water are not bad for grout.
I sealed a floor area real well (20 x20 porcelain). Later it
flooded for a day and the people told me it took weeks for
the grout to dry out. My opinion is that the water got under
the sealer and into the grout, and took a l-o-n-g time to dry
All this is my opinion of course...