It is not really necessary to seal slate tile, but many people prefer to do it to add a finish to the floor or to protect the slate. Sealing slate tile properly will keep it looking nice for years, and will keep you from having to do other maintenance to the tile later, such as fixing cracks or cleaning stains. When sealing slate tile, you must decide what type of sealer you want to use, and whether you want a matte or a glossy finish for your tile.
After the Tile is Laid
Once a tile floor has been laid, the mortar should be allowed to dry and to cure before the tiles are grouted. Mixing a penetrating sealant with your grout, instead of water, with greatly increase the bonding strength of the grout.
This will strengthen the bond between the tile and grout, but will also improve the water resistance of the floor. This is especially important if you are installing slate tile onto concrete slab, which can draw moisture from the ground into the floor. Sealing slate tile is more necessary in that situation.
After the tile has been grouted, it should be left to dry for about a week before anything else is done. After a week, the tile should be cleaned thoroughly. It should be washed several times to remove all grout remnants so that it will not become sealed onto the tile.
The floor should then be allowed to dry before you begin sealing slate tile. Sealants can not be applied on tile that has been waxed or oiled, and it can not be applied if the tile is still damp.
Choosing a Sealant
To choose a sealant for sealing slate tile, you will need to decide what it is you want from a chemical sealer, and how you want the finished tiles to look. If you want a shinier look, there are glossy sealants, and there are also popular sealants that will give the tile a wet look without going glossy. A matte finish is probably best if you want the tile to retain as much of its natural look as possible.
There are two main types of sealers; topical tile sealers, which stay on the surface, and penetrating sealers, which penetrate further into the tile to give it better protection.
Water-based polyurethane sealants have one of the lowest levels of toxic fumes, and are a popular choice for that reason. It is also non-cracking and non-yellowing, leaving the tile in good condition for years. Products for sealing slate tile can usually be bought at retail stone dealers, or at major home improvement stores.
Applying the Sealant
When you are sure that the tiles are clean and dry, you can begin the task of sealing slate tile. The sealer should be applied in a thin coat that follows the tile grain. After letting the tile dry for at least two hours, a second thin layer should be applied. Check the label to be sure how long your sealant requires to dry.
While sealing the tile, make sure to have some form of ventilation in your work area, such as an open door or window. Even the safer sealants with have some level of volatile organic compounds, so caution should be used.