As I have repeatedly indicated, the interaction with a professional tile distributor, will be the best way of identifying which tile, stone or other hard surface material, should have a sealer applied, after the installation of the material. If the tile is glazed, then the only surface requiring a sealer would be the grout in the joints. In most cases, this is not recommended. If the surface material is not glazed, and is absorbent, the use of a sealer is normally required. Absorbent tiles, stone or other hard surfaces, will stain, if liquid is dropped on the surface, and will not provide an adequate surface for continuous use. An absorbent surface is a difficult surface to wash, remove stains and maintain. The sealing of these types of surfaces is recommended. In some instances, both a applied staining and sealing will be recommended. If this is the case, a professional installer is recommended to obtain the proper result.
Sealers for tiles will stop any surface liquid from seeping into the surface of the material; however, caution should be taken to ensure that the sealer is compatible with the surface that the sealer is being applied on. Sealer incorrectly applied, or applied to the wrong surface, could spall off, or flake, which will require the incorrect sealer to be stripped from the top of the tile surface.
Unless you have been professionally instructed by a dependable distributor, contractor or tile consultant, do not apply a sealer on the surface of your tile. It is important that you are sure that the sealer will enhance the surface of the product and not cause maintenance issues in the future.
In addition to ensuring that the correct sealer is being applied, the surface of the tile, stone, brick that the sealer is being applied to, must be clean and free of dirt and impurities. If the surface is not clean, all you are doing is sealing in the imperfections on the surface of the product.