Sealing Natural Stone
Several factors must be considered prior to determining if the stone should be sealed:
• What is the hardness, density, and durability of the stone?
• How porous is the stone and how fast will it absorb a liquid (also referred to as the absorption coefficient)?
• Is the stone expected to be in frequent contact with a staining agent?
• What type of finish was applied to the surface?
For example, a polished surface is more resistant to staining than a honed surface.
• Will the sealant affect the color or other aesthetics of the stone?
• If a resin was applied to the stone, how will the sealant react with the resin?
• Where is the stone located (e.g. countertop, floor, wall, foyer, bathroom, etc.)? Residential or commercial?
• What type of maintenance program has the stone been subjected to?
The type of stone, its finish, its location, and how it is maintained all need to be considered when determining how to protect the stone.
In some cases it makes sense to seal the stone. Once properly sealed, the stone will be protected against everyday dirt and spills. In other cases, it is best to leave the stone untreated. Topical sealers can alter the surface texture and finish as well as build up on the surface, creating a layer that is less durable than the stone. Generally, topical sealers are not recommended in exterior applications because they can trap moisture within the top layer of the stone, which may lead to surface deterioration during freeze/thaw cycles.
The Marble Institute of America’s position on sealers is as follows:
The Marble Institute of America (MIA) recognizes the benefits that sealers can provide in certain applications. MIA recommends that care be exercised in the application of any chemical to a stone’s surface. Although normally innocent in and of themselves, some sealers have reportedly reacted with some cleaning/maintenance chemicals and/or with components within the stone surface, causing some reactions.