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Dec 25, 2020/Stone Care Guides

A Deep Dive into the World of Stone Sealers

Stone floor restoration and sealer

The refurbishing of outdoor installations like stone facades, statues, stone bridges, and even tombstones has long been a stone refurbishing activity held in the highest of regards and been one of the more recognized examples of the craft. As you might expect, a great deal of skill and expertise goes into this kind of work. From specialty cleaning to pointing, and even as far as replacing pieces of stone in the feature itself. When dealing with something as visible and recognizable as a statue, facade, or bridge, great care must be taken not alter the features of the installation in the course of repair.

As such, it would be a disservice to the craft as a whole to attempt to explain everything in a relatively short blog post, which is why we're not going to do that here. Instead, this post will focus on the less demanding tasks that are often required of a stone restoration contractor.

By far one of the largest aspects of a stone restoration professionals’ job will be the cleaning and sealing of stone installations, with the majority of these installations being floors.

It may seem odd-even demeaning-that a highly skilled stone restoration professional would take on the seemingly simple task of cleaning and sealing, but the truth is this very much within their wheelhouse, so to speak, and it is not an unskilled task. Far from it, in fact, as it will usually help to reinforce the contractor's reputation, not to mention giving them more exposure to the market. Word of mouth still counts for a lot, even in today's social media-fueled world.

The important thing to remember is that natural stone is not cheap-stone installations even less so-so people are more hesitant when it comes to hiring someone to tend to their installation. Cleaning and sealing may be one of the less taxing activities a stone restoration professional will undertake, but that is not the same as an unskilled job. As such, it provides a perfect inroad for a contractor to build up their reputation.

Stone Sealers

There are two main types of stone sealers that are distinguished by the way in which they go about their task. Topical hard-shell sealers act as a kind of coating, largely sitting on the surface of the stone. Penetrating sealers, on the other hand, impregnate the stone. There are other types of sealers, such as soft-shell topical finishes, but these are so rarely used that we won't concern ourselves with them here. It should be noted, however, that a significant portion of the reason these types of sealant are not commonly used is not necessarily a reflection on their effectiveness, but on certain misconceptions about them. If people generally believe a product to be inferior, they won't request it, even if they are wrong. Of course, stone restoration contractors are often likely to know better, but that does not mean they are interested in spending time convincing a potential client rather than just taking the job. It is worth remembering that the world of stone restoration is often an expensive one, and the types of personalities a contractor will encounter are not likely to be the kind of personalities who are interested in having what they asked for questioned. In other words, pushing the virtues of something like soft-shell sealant on a client who wants hard-shell could well lose them the job.

Hard-Shell Topical Sealers

Mostly based on urethane resins, hard-shell topical sealer should only really be used to coat problematic stones, such as slate, and sandstone. The type of protection that this kind of sealer offers is more familiar to clients, as it is similar to what they are used to with wood products, such as furniture, and floors. There are problems, however. Two of them, in fact.

The Natural Cleft Problem

The typical finish of the kinds of problematic stone that suit this sealant is not even, and features what is known as a natural cleft. Whereas wood can be sanded smooth, the process of grinding the natural cleft from something like slate so that it is flat would defy the point of using that kind of stone in the first place, since it is that natural look that is appealing. But what is the problem with the natural finish of the stone?

Well, it depends on the use, but for the more common uses, such as floors, the high spots of the stone would be subjected to much more wear and tear than the low points. This would result in an accelerated wearing down of the surface, which would eventually expose unsealed stone. Not the best look.

Of course, if the stone is being used somewhere where it will not be subjected to things like foot traffic, this would not be a problem.

The Long-Term Care Problem

Over enough time, any surface will wear down. In the case of an urethane finish on a wood floor, this can be dealt with by sanding the wood back to a smooth finish and resealing it. With a stone surface like slate or sandstone with a natural cleft, the only way to achieve this would be to grind the stone flat. As we mentioned above, this is possible, but it would defy the point of having such beautiful and interesting-looking stone in the first place.

This leaves only one way to remove the topical finish without affecting the surface of the stone itself; chemically. Unfortunately, urethane sealers are not as easily stripped away as the unpopular soft-shell options. The options going forward do not make life any easier. The solvent, methylene chloride, is effective, but is rumored to be carcinogenic. There is also a product made in Richmond, Virginia, that can allegedly perform this task and is non-toxic, but we are given to believe it is very slow when compared to the methylene chloride, and requires several applications to get the job done.

This list of problems tends to put many contractors off of taking on a job like this, especially since there is so much other business out there at the moment. Still, the cost of any work will usually be determined by a compromise between the most a client is willing to pay and the least a contractor is willing to accept. If a client is struggling to find anybody to take on their work because of the problems we have covered here, they will likely be willing to pay more to the contractor who will take the job on.

Hard-Shell Conclusions

All things considered, hard-shell topical sealers certainly have their place in the world of stone sealing, but that place is apt to be one with low traffic. While they are very effective, the longer term ramifications of this type of sealant lead many contractors to go a different route.

In situations where there is not much traffic, there are far fewer downsides to hard-shell topical sealers. But for those high-traffic areas, it is often a case of trading one problem for another. And, with that in mind, let us consider the alternative...

Impregnating Sealers

Impregnating sealers can be broken into two major types; solvent-carried and water-carried. Solvent-carried impregnators can be based on siloxane, silane, or ester epoxy-based. Water-carried impregnators, on the other hand, are based on resins in the fluorocarbon group, such as alphatic resin, though there are some silicon-based options available. There is also talk of research into nano technology-based alternatives, though there is nothing concrete to speak of yet.

So, how does an impregnator work? Rather than forming a protective coating on the surface of the stone, impregnating sealers work by being absorbed into the stone. Once absorbed, the carrier (the solvent or water we mentioned above) evaporates, leaving the resin behind to clog up the pores of the stone.

The way this kind of sealant works means that it is only effective on more porous types of stone that it can be absorbed into. Technically speaking, all dimensional stones are porous to some degree, but some are so dense as to be practically water-tight, which is not good for this kind of sealant. Attempting to use this type of sealant on stones that are too dense to absorb it will not only not have any benefit, but it can also create "ghost water stains" as a result of the sealant.

As a general rule of thumb, if the absorbency rating of a stone is less than 0.2%, sealing is not advised. Of course, finding out the absorbency rating of stone is not always possible, and certainly not easy. Another way to check is to perform a test. Find a piece of stone like that which is to be sealed that can be discarded. Lay it flat and drop a little water and a little cooking oil on, and let them sit for five minutes. Then wipe the stone clean. If there are dark patches where the water and oil were, the stone has absorbed some liquid. In this case, sealing the stone would make sense, since you know it will absorb the sealer. This is generally the best way to determine if a floor should be sealed (if you can get a piece of stone to test on), especially if the installation being sealed is an old stone structure. 

It is also worth remembering that the absorbency ratings for any given type of stone are determined when the stone is in its pristine state, which may not be accurate even for relatively freshly cut stone, never mind stone that is years, decades, even centuries old. There is also the matter of how the stone is processed. Some stones are cut or polished in such a way that it effectively increases the surface tension of the stone, meaning it would not be suitable for sealing, even though its absorbency rating would suggest otherwise.

Another factor is the environment in which the stone resides. For example, an absorbent stone that is installed in a kitchen environment will certainly need sealing, as there is a high likelihood of spillages and general moisture content in the air. Conversely, stone that is installed on the walls in the confines of a dry office is unlikely to suffer much in the way of spillages, so the practicality of sealing it would need to be considered more carefully. Which brings us nicely to our next topic.

Does Stone Need Sealing?

More often than not, the question which sealer should be used on a stone installation is asked without ever considering if the stone needs sealing. As a general rule, stone is better for being left alone wherever possible. Granted, there are situations where sealing stone is necessary, but there are also situations where it is emphatically not necessary, and, in those cases, we would recommend leaving the stone be.

So why do stone installations get sealed so often if it is not essential? No doubt fuelled by the people and companies that sell sealant, there is a sort of societal lionizing of sealant as some magical substance that can protect everything it comes in contact with. In truth, impregnating sealers are not nearly as necessary as people tend to believe, nor are they as effective as the marketing would have you believe. Staining is certainly a possibility with a stone surface-particularly a porous one-and sealer can help in the right circumstances, but the truth is staining is often not that big a deal, and can be removed without too much fuss.


All of that being said, there is the matter of practicality to consider. Yes, sealing is not as necessary as many people believe, and soft-shell sealer is a perfectly viable solution in those cases where it is necessary. But given how much of a stone restoration professional's work is rooted in cleaning and sealing, should those same professionals really be pushing their clients away from it?

Unfortunately, though you may feel you are doing your customers a favor, you are just as likely to lose work while a potential client finds another professional who is more willing as you are to receive gratitude over saving your client money.

What you can do is go with the flow, but ensure your clients get the best possible job in the process. They will get what they are really seeking-peace of mind-and you will continue to build your reputation.

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