Constant exposure to water, and settling or shifting of the shower walls can cause the grout to crack around tiles. Cracked grout must be repaired to keep water from seeping through the cracks. Over time the water will cause damage or rotting to the underlayment. This will lead to costly repairs that could have been avoided by repairing the cracked grout. Grout repair is a job that you can do yourself without hiring a tile installer.
Cut the grout lines with a grout saw. A grout saw is a handheld tool that has a narrow diamond carbide blade. The blade is designed to fit into grout lines between the tiles. Pull the blade back and forth to remove as much of the old grout as possible.
Suction up the grout pieces with a shop vacuums, and wipe the dust off of the tiles with a damp rag.
Fill the 1-gallon bucket with water to a depth of no more than 1 inch. Pour matching non-sanded grout into the bucket a little bit at a time. Mix the grout into the water with the 3-inch putty knife. Continue adding grout to the water until it is slightly thicker than a brownie mix. The grout should be thick enough to stay on the putty knife, but not so thick that you cannot spread it.
Spread grout into the cut out spaces between the tiles with a grout float. Do not grout over a 3-by-3 foot area at a time. Hold the float at a 45-degree angle and drag it across the tiles. Scoop up excess grout with the putty knife, and put it back in the bucket.
Fill the 5-gallon bucket with warm water. Dip the sponge into the water and wring it out. Hold the sponge flat to the tile over the grout lines. Apply light pressure to the sponge and pull it across the grout lines to smooth them out. Pull the sponge horizontally across horizontal grout lines, and vertically across vertical grout lines.
Rinse the sponge often, and repeat steps five and six to finish grouting.
Regrout the entire shower if the grout is old and stained. Cut the grout lines with the grout saw deep enough so that they will hold new grout. You can use a 4 1/2-inch grinder with a diamond carbide grinding wheel as long as there is enough room between the tiles. Be very careful if you use this method to not get too close to the tile. You will crack them.
Do not grout more than a 3-by-3 foot area at a time. Non-sanded grout dries quickly and it is extremely hard to remove after it has dried.