What happens if sealer goes on wet pavers?

ASCC, writing about decorative concrete, names exactly this moment: “Following a wash, there may be a significant amount of water left in the concrete even though the surface may appear dry.”

A sealer applied on that look is applied on the surface's appearance, not its condition. How long pavers need to dry first is answered under how long pavers need to dry before sealing; what follows here is what happens when sealer meets the water that is still there.

Euclid Chemical names the mechanism for a solvent based concrete sealer applied to a wet surface: “the sealer will not bond to the concrete surface but will instead float on a trapped film of water.” ASCC, writing about sealers on concrete generally, says applying sealer to wet concrete can make the surface fog up or blush, turning it white, and can cause adhesion problems, and that sealer applied over a dew covered surface can fail to properly adhere.

The milky-white result has its own name, defined under blushing. Moisture at application is one of the four whitening conditions ICPI lists for sealed pavers, and the full list is under the paver cleaning page's whitening section.

Not every white cast comes from a wet surface. ASCC notes that efflorescence and sealer whitening are commonly mistaken for each other. A line that runs only along an edge has its own sourced causes, under why a white line shows at the edge of sealed pavers, and a surface that simply looks washed out is taken up under why pavers fade.

Techniseal answers the question for its own paver Protectors in its product guide. Asked whether a Protector can go on while the surface is still humid, it says no, even with a water-based Protector: “The Protector could react with moisture and leave a white film on the surface.”

A water based film has a second moisture problem, one that sits in the air rather than on the pavers. Euclid Chemical, writing about concrete sealers, says water based acrylics dry by coalescence, and “If the temperature during application is too low or humidity is too high, the coalescing solvent will evaporate before the water does and the sealer will dry white or powdery.” ASCC's troubleshooting guide files both kinds under a single sealer whitening entry.

The makers put the prevention on the product. SEK-Surebond's SB-1300 data sheet says “Never try to seal wet or damp surfaces,” and ICPI's Tech Spec 5 asks, for most sealers, for at least 24 hours without moisture or surface dampness before application. Why that sits between the clean and the seal as a condition rather than a date is set out under sealing as a sequence with a condition between the steps.

For a specific product, its own label carries the dry condition, and a film that has already whitened is a question for that sealer's maker.