Some of the grout in our guest bathroom had been cracking. We had the tile installed less than a year ago, so needless to say, I'm not happy at all that it is already in need of repair. Here are a couple of pictures of the cracked grout. What you're looking at is where the tile on the floor meets the tile on the wall.
The crack is more obvious in the top picture. In the lower pic there is a crack in the grout near the door frame.
Here are the supplies that I needed. At Lowes, I bought sanded caulk (AKA grout caulk) and a little tool to smooth the caulk out. A got a piece of chipped grout from the bathroom and put it in a ziplock bag and took that to Lowe's with me. The sanded caulk comes in different colors, and I wanted to get the closest color match that I could. The sanded caulk feels a lot like grout when it dries, but it is kida rubbery like regular caulk. We already had the caulk gun and screwdriver pictured.
I used the screwdriver to chip the damaged grout away, then I got a hand vac to clean up the mess. Then I applied the sanded grout. It was a messy job. I kept two old washcloths and and old towel within reach to clean up mistakes as I went. I used nail polish remover to clean up any smudged caulk, and it worked nicely. Here's the finished product:
I had to remove the air vent while I worked, but it just slips in and out.