When Not to Use a Sealer on Stone Surfaces
By Fred Hueston
Applying a sealer to a natural stone surface can be a critical step in protecting and preserving the material—but sealing is not always appropriate. In fact, improper or unnecessary use of sealers can cause more harm than good, especially on certain types of stone and in specific environments.
As a stone restoration professional, it’s your responsibility to know when not to use a sealer—whether you’re working indoors or out, on honed limestone or polished granite. This article outlines the key situations and considerations where sealing should be avoided or approached with caution.
1. Avoid Sealing Stone That Contains Moisture
One of the most common mistakes made in stone care is attempting to seal a damp or moisture-laden surface.
Even if the stone looks dry on the surface, moisture may still be trapped within the pores. Applying a sealer or impregnator under these conditions will prevent proper absorption and bonding, potentially leading to:
- Blotchy or uneven appearance
- Premature failure of the sealer
- Trapped moisture, leading to efflorescence or spalling
Always test for moisture content using a moisture meter, especially on recently installed stone or surfaces exposed to the elements. If there’s any doubt, wait.
2. Be Cautious When Sealing Outdoor Stone
Outdoor environments introduce a host of challenges—UV exposure, rain, freeze-thaw cycles, and heavy traffic. Some restoration professionals instinctively reach for sealers to protect outdoor stone, but this should be done with care.
Consider the following:
- Sealers that form a topical film can trap moisture beneath the surface, especially in rainy or humid climates. This can cause flaking, peeling, or stone degradation over time.
- Many impregnating sealers are not UV-stable, and their performance can degrade rapidly when exposed to constant sunlight.
- Breathability is crucial outdoors. If the stone cannot “breathe”—that is, allow vapor to pass through—it may deteriorate from within due to trapped moisture and temperature extremes.
In general, select breathable, UV-resistant impregnators only when sealing is absolutely necessary—and never seal outdoor stone just for cosmetic reasons.
3. Highly Polished Stone May Not Need Sealing
Some dense, polished stones—such as high-grade granites or certain marbles—are so low in porosity that a sealer won’t absorb into the surface.
Attempting to apply a sealer here often results in:
- Streaking or hazing
- Sticky residue that attracts soil
- Wasted product and unnecessary labor
Before sealing, always perform a porosity or water drop test to determine if the stone will absorb the sealer. If the water beads up and doesn’t penetrate, sealing may not be required—or may even be detrimental.
4. Sealing Can Alter the Aesthetic—Not Always for the Better
Certain sealers, particularly enhancing sealers, will darken the stone or enrich color tones. While this can be desirable in some applications, it may not match the client’s expectations or the surrounding design.
Additionally, if the sealer is applied unevenly—or if the stone has natural variations in porosity—the result can look blotchy or stained. Always test the product in an inconspicuous area or use a mock-up before full application.
5. Routine Maintenance Might Be Better Than Sealing
In some cases, the best protection is not a sealer, but simply a sound maintenance plan. Frequent cleaning with pH-neutral stone-safe cleaners and the use of mats or runners in high-traffic areas can preserve the stone’s finish without altering its natural breathability or appearance.
Remember: not every stone needs to be sealed, and over-sealing can cause just as many problems as under-sealing.
Conclusion: Know the Material. Know the Environment. Know the Limits.
Sealers are valuable tools in the stone professional’s arsenal—but they are not cure-alls. As a restoration specialist, your expertise lies in making educated decisions, not automatic ones.
Before sealing, ask:
- Is the stone dry—inside and out?
- Is the environment suitable for a sealer?
- Is the stone dense enough to warrant sealing?
- Will sealing alter the look in a negative way?
- Is routine maintenance a better option?
Knowing when not to seal is just as important as knowing how to seal.
