Although many man-made tiles such as ceramic and porcelain are porous and do not soak up water, the grout joints between the tiles are still made up of concrete, which is readily porous. Natural stone tiles are also more porous than man-made. When finishing a backsplash installation, it is a good idea to seal the area to protect it from the water of the sink and general cleaning.
If you are dealing with a small-tile installation where the joints are less than 1/8 inch and you only have a small area to deal with on man-made, glazed tiles, consider sealing only the grout joints as opposed to the entire backsplash installation. The tiles themselves do not need any protection. Rather than use a paintbrush, roller or sprayer to apply the sealer, look for the applicator-tip sealers sold at home improvement stores and use a tip that fits your grout joints. Simply apply the sealer directly to the joints. For wider joints on ceramic and porcelain tile installations, use a paintbrush or foam brush to apply directly to the joints.
In the case of natural stone tiles where you want to seal the entire installation surface, not merely the joints themselves, consider a paintbrush, roller or spray-on application. Spray-on applications are the best if you have a countertop as well as the backsplash to cover, and when you are sealing the tile for the first time, ahead of the sink installation. If the sink and other components are already installed, use a paintbrush or roller to apply the sealer. The stones readily soak up excess, but if you do have runs during the initial application, merely spread the sealer out across the tiles with a sponge to allow the tiles to absorb the extra.
As a general rule, sealer isn’t a chemically marring product that harms other material surfaces, but you should always be safe as opposed to sorry. Use masking tape to cover all the edges around the countertop and underside of the upper cabinets to ensure that you do not apply sealer onto any surrounding installations. When you are finished sealing, remove the masking tape. Drape plastic sheeting over areas such as the kitchen sink or the stove when you are applying sealer above appliances.
Always apply sealer in an open, well-ventilated area. For kitchens, make sure the outdoor windows are open and all doorways out of the kitchen are open. Wear rubber gloves, safety goggles and a respirator mask when applying sealer, as the chemicals within can cause allergic reactions with some individuals. If you are working with older tile that you are resealing as opposed to sealing for the first time, clean the surface of the tile and the grout joints with a tile cleaning solution available at home improvement stores. Always follow manufacturer instructions for application specifics and drying times.