I’ve used this on my own bathroom floor, which has a more complicated hexagon pattern, so it took a bit of time. But I just listened to an episode or two of a podcast and got to work! I recommend doing two coats.
Promising review: “We moved into our new home, and the shower had a really yellow grout. Cleaning didn’t do anything, and I saw this grout pen online. What a saver! Otherwise, I would have just hired someone to replace my shower. I used this grout pen, and my husband was so impressed 😀 The tip loses its shape fast, but just pull out the TIP AND FLIPPED IT OVER, and you’ll have a new tip.” —jennifer
Get it from Amazon for $8.99+ (available in two sizes and three colors).