The process and techniques involved in grouting an outdoor stone installation are no different than an indoor stone or tile installation. That said, a specific type of grout must be used, and the tile needs to be prepared properly. For this task, you will need grout, water, a padded grout float, a grout sponge and two 5-gallon buckets. For grout to maintain its appearance and durability, it must be applied to tile that is installed on a proper subfloor, such as concrete or plywood, and attached to that subfloor with the proper adhesive.
For outdoor environments, an epoxy-based, sanded grout must be used since its properties and durability will be able to handle the stresses of an outdoor setting. Epoxy grouts are more water-resistant and can withstand differences in temperature better than other types of grout. Sanded grout includes -- as you may have guessed -- sand, which adds strength to the mortar contained in the grout. It is able to create a strong bond in the wider joints and gaps that patio surfaces most often have. Epoxy-based sanded grouts are available in just about any color to match your stone.
Prepping the Tile
Many types of outdoor stone are made of porous materials like slate, marble, terracotta and limestone. These tiles must be sealed first with a penetrating sealer before grouting. Failing to do this will possibly result in a fogged effect on the tile from the grout staining the stone. Before grouting or sealing the stone, the area should be swept clear of debris and dust.
Preparing the Grout
The amount of water needed to mix your grout will be outlined on the packaging that it came in and can vary by manufacturer. When mixing your grout, you should not add all the water at once but start by adding about half and mix the rest into the grout in increments. Mixing grout can be done with a trowel, or you can use a grinder tool with a mixer attachment.
Grouting the Patio
Grouting is done in small patches of just a few square feet. You should start as far as possible from any doorway that you need to access so you do not trap yourself outside. With your padded grout float, scoop a cup-size portion of grout and place it on the surface. With your float at a 30-degree angle, spread the grout across the surface, diagonally to the joints. Grout should be spread evenly, so keep your eye out for any gaps in your grout coverage. After you have applied the grout to an area of a few square feet, clean the surface. With a wet grout sponge, clean the surface in circular motions, diagonally to the joints in the tile. Rinse the sponge often. After your have cleaned the surface of the tiles, run the sponge along the joints to form and recess the grout in the joints. Repeat this process across the floor.
Stay off the floor for at least 24 hours after grouting. After the grout has finished drying you may notice that the tile appears hazy. If mopping the floor does not remove this, you probably have remnants of the grout and its inherent adhesives stuck to the surface of the tile. This is caused by not properly cleaning the surface of the tile with your grout sponge. There are products called grout haze removers for removing this from the surface.
Sealing the Patio
Because the grout will be outside and exposed to the elements, it should be sealed. If you do not seal the grout, liquids and other soils may stain the material over time. Penetrating sealers are available at most hardware stores and tile shops. Applying it to the grout is a simple process; use a foam brush to apply the sealant to the grout. Sealing should be repeated once a year.