Until very recently, tiles needed to be installed with a minimum of a 1/16-inch grout joint. This joint was required not only to help seal out moisture, but to prevent damage from tiles rubbing against one another and to make up for uneven tile sizes.
New technologies have helped to produce a method of tile production that uses compressed clay to produce tiles that are extremely even in size with crisp, straight edges. These tiles, known as vitrified tiles, can be installed with no gap or grout joint between them. If this is a drywall application, such as a kitchen backsplash, you may not wish to grout this tiles at all. If this area will be subject to water or moisture, you will still need to grout the tiles, despite their lack of joint. Unsanded grout can be used to pack these tiny gaps, sealing the tiles.
Mix the unsanded grout to the consistency of slightly runny peanut butter, according to the manufacturer's instructions. Unsanded grout is made up of extremely fine particles of cement and can be pushed into the space left between tiles that have been butted up against one another.
Scoop up some grout with the flat of the grout float and begin to scrape it over the tiles' edges. Press the grout float onto the tiles' edges first on its edge, and then with the flat side of the float. This will pack the joints with the unsanded grout. Because there is not a large space, you will be able to scrape up the grout with the float and continue using the same scoopful for an extended period of time. The grout will seek out any gaps and fill them, no matter how small. Just be sure to use some pressure as you apply the float to push the grout in tightly.
Wipe up the excess grout from the tiles with a damp sponge after about 10 minutes. This will give the grout time to set in the joints, but not enough time to dry on the tiles. Be sure not to use a lot of water, because this could wash the grout back out of the joints before it dries. Allow the grout to dry completely for 24-hours.