The health, or floor section directly in front of the fireplace, is particularly prone to staining from soot and ash. A natural stone hearth, either in slab or tile form, can be permanently stained if not sealed on a regular basis. Because natural stone is made up of numerous, microscopic pores, ash, soot and other stains can go below the surface of the stone where they cannot be wiped away. A good impregnating sealer fills those small holes to keep stains on the surface where they can be easily cleaned off.
Clean it Properly
Before the stone can be sealed, it needs to be clean and free of any ash, soot, oil, dust or other debris that could be sealed into the surface of the stone. Use a stone cleaner and a soft cloth to clean the entire surface of the hearth. A cleaner designed for stone will also help prolong the life of the seal. Surfactants in soaps can break down the seal over time, which can leave the stone vulnerable. A neutral PH cleaner designed for stone will not only clean the stone prior to sealing it, it will help to protect the stone over time.
Choose the Right Sealer
There are many types of natural stone. Granite is harder and denser than marble, which is harder and denser than limestone. Make sure you use a sealer designed for the stone you have. Sealers come in both water and silicone bases. A water-based sealer has fewer VOCs, but will not seal as well as a silicone-based sealer. For extremely porous stones, like limestone, special, deep-penetrating sealers made of silicone are necessary to fill all the holes. Granite, however, can frequently use a less intense sealer and still achieve the same protection.
Apply it Properly
After cleaning the stone, allow it to dry completely. An unsealed stone will absorb some water or moisture during cleaning. This will temporarily darken the color of the stone. Wait until the stone has returned to its natural color before sealing to ensure the pores are open and ready to receive the sealer. Paint the sealer on with a foam paintbrush. Overlap each stroke slightly, and fully saturate the surface of the stone. Look at the hearth from an angle to check for dry areas and ensure that you don't miss any spots. Let the sealer penetrate the stone for 10 minutes, then buff away the excess with a soft cloth. If the sealer dries for longer than this, apply additional sealer. The sealer will emulsify itself, allowing you to buff it away in a few minutes.
Reseal as Needed
The length of time that you need to go between sealing applications varies from hearth to hearth. The type of stone, the type of sealer you use and the type of cleaner you use will all affect how soon the stone will need to be resealed. To check the seal on the stone, sprinkle some water on its surface and wait a few minutes. On well-sealed stone, the water will bead up on the surface of the stone and will not darken where the it was applied. A stone that requires a new coat of sealer will absorb the water. Check the hearth a few times a year for the first year to determine how often you need to reseal, until you know approximately how often it will need to be required.