The Truth about Stone Sealers and Impregnators

I receive several calls a week about the use of impregnating sealers on outdoor stone and in interior wet areas like showers.

The primary question: Should stone be sealed in these conditions?

The answer is no.

Outdoor settings and showers are exposed to unregulated humidity and temperature fluctuations, resulting in water vapour in the air. Temperature along with humidity can also result in condensation whereby the water vapour in the air becomes liquid.

Why does this matter?

Impregnators or penetrating sealers are designed to permeate the stone’s surface and deposit solid particles in the stone’s pores or coat the individual minerals below the surface of the stone. This restricts water, oil and dirt from entering the stone. However, most impregnators on the market today, whether solvent or water-based, are breathable. This means that while the stone will be protected from water entering its pores, water vapour may pass through.

You may think water penetration is a positive. However, once the vapour enters the stone it can condense and become a liquid. Since impregnators protect against water in its liquid state it becomes trapped within the stone’s pores and will not escape until it evaporates or turns into a vapour.

Trapped water can result in a slew of problems. Stones with iron content can begin to oxidize, and natural salts within the stone can dissolve and cause pitting and spalling. As well, the stone will appear darker since it is constantly wet.

Now that you know where not to use impregnators, it is important to understand the difference in the terms used to describe these sealers to clear up any confusion in the flooring industry.

A sealer is defined as a coat (as of size) applied to prevent subsequent coats of paint or varnish from sinking in. In other industries, such as cleaning and janitorial, a sealer is similar in that it is a coating that is placed on the surface of a substrate to seal it so something else can be applied. In the stone industry, a sealer is also defined as a coating. The proper description in the scientific community is ‘film former’ because when applied it forms a film on the surface of the stone.

Impregnate means cause to be filled, imbued, permeated or saturated. In the stone industry, the same definition applies. Impregnators penetrate below the surface of the stone and protect from within. There is no coating or film formed on the surface of the stone.

Penetrating sealer is really an oxymoron if you go by the pure definition of each word.

If a sealer is a coating that sits on the surface, then how can it also penetrate the stone?

This term, however, is often used interchangeably with impregnator. Technically, you would expect a penetrating sealer to pass below the surface and also form a film on the surface of the stone. But while there are products that can do both, most in the flooring industry do not.

Frederick M. Hueston is the founder and president of Stone Forensics, which has gained national renown for providing knowledgeable stone, tile, concrete and masonry consultation services, including inspection and failure analysis, historic preservation and consultation services. In addition, Stone Forensics provides training and education to professionals in the stone industry regarding marble and other natural stone, tile and terrazzo surfaces.