The only problem I see with this method is the amount of grout being pushed into the crevices, and it not applying a strong enough presence and seal against any spills that may occur from time to time.
I thought the whole reason for applying grout in the first place was to create a seal between the tiles, and to give the tile more stability around the sides to prevent them from moving from side to side. Using a caulk gun would only place the grout in the gap, not fill it with enough pressure to completely fill any air pockets around the tile. I would also point out that grout is pretty thick, and pushing back that metal plunger on a caulk tube can be very difficult, and once refilled, would be a bit difficult to push on with that thick grout in there. Am I wrong here? Victor Gouveia
