Why can pavers look dry and still be too wet to seal?

Concrete is porous, and a cleaning pushes water into it as well as across it. The surface is the part that meets the air, so it is the part that dries first, and its color is the only thing the eye can read.

The American Society of Concrete Contractors' Decorative Concrete Council puts the gap in one sentence in its sealer guide: “Following a wash, there may be a significant amount of water left in the concrete even though the surface may appear dry.” The guide is written about decorative concrete sealers, not about pavers, and it is quoted here as a concrete document. Concrete pavers are concrete too, and the same reasoning about a surface that dries ahead of what is under it applies to them, though the guide itself does not speak to pavers.

The sealer makers write their condition in words that go past color. Techniseal's application conditions for its WL1-2 and NS Protectors start with “Surface must be clean, dry and warm to the touch.” Dry and warm to the touch are two separate items on that list, and neither one is “looks dry”.

ICPI does not stop at the paver face. Its Tech Spec 5 list of conditions under which pavers may draw efflorescence to the surface, or a sealer may whiten, includes “The surface and joints are not dry.” The joints are named on their own.

The reasoning is simple. The joints are sand, packed between the units and open at the top, and a rinse fills them the way it wets the faces. Sand in a narrow joint has little surface exposed to the air, so it can hold water after the broad faces on either side have dried. ICPI's cloudiness sentence names the result: when the base drains poorly or “the sealer is applied to saturated sand in the joints, or is applied too thick, the sealer can become cloudy and diminish the appearance of the pavers.”

The American Society of Concrete Contractors describes a related fault for decorative concrete sealers put on wet concrete, a surface that can fog up or turn white; the glossary covers that white haze under blushing. The full list of whitening causes, and ICPI's remedy, is on the paver cleaning page where it explains why sealed pavers turn white or cloudy, and the finished look of sealer that met a wet surface is covered under what happens if sealer goes on wet pavers.

None of the documents explains why it counts hours, but the difference is practical: elapsed time can be counted from the driveway, and water under a dry-looking color cannot be seen.

ICPI's wording is a span, not an appearance: “Allow at least 24 hours without moisture or surface dampness before application.” The two documents behind the 24 hours, and the numbered steps SEK-Surebond publishes, are set out under how paver sealing works, step by step.

The count is also fragile. A shower partway through the wait wets the surface and joints again, a question answered under whether rain restarts the clock before sealing, and a section of pavers out of the sun is its own case, covered under whether pavers can be sealed in the shade.

None of the documents quoted here gives a homeowner a test for moisture inside a paver. What they do say points to a few things that can be noticed without any tool.

• Rain or a sprinkler cycle since the cleaning, the case taken up under whether pavers can be sealed if the sprinklers run at night.
• Joints that look darker than the paver faces around them, which can mean the joint sand is still holding water.
• Low spots where rinse water collected, and edges where the drive meets a lawn or a bed that was watered.
• Sections that stay out of the sun, which tend to dry more slowly than the open drive.

Past those, the product decides. How dry is dry enough is written on the label of the sealer being applied, and it is not the same on every label: Techniseal asks for clean, dry and warm to the touch, and the few labels that allow a damp surface are taken up under whether any paver sealer can go on damp pavers right after cleaning.