Why is there a white line at the edge of my sealed pavers?

No maker or industry guide cited on this page names an edge line as such, so the starting point is where the line runs. A line along a border cut in by hand, around a planting bed or a wall, fits one mechanism; a band a few pavers wide fits another; a line that traces the joints fits a third.

ASCC, writing about decorative concrete, describes the first: "If a section is cut in and allowed to dry, there will likely be a visible line between the cut in area and the rest of the sealed concrete." ASCC calls it a visible line rather than a white one, and a wet edge between the two areas is what prevents it. Techniseal writes the same instruction into the application steps for its NS Protector: "Maintain a wet edge to avoid overlapping marks," and for later sections, "maintaining a wet edge to prevent overlap marks." Marks from overlapping passes across the open field, rather than at one border, are covered on overlap and roller marks across a sealed field.

The second mechanism is thickness rather than timing. Techniseal's NS data sheet: "An over-accumulation of product could lead to surface whitening." ASCC also lists whitening among the problems over-application causes. Where a border gets an extra pass, the coat there can end up heavier than across the field; ASCC also notes that sealer allowed to puddle or overlap dries darker, so a heavy band is not white by rule. What over-sealing looks like more broadly is covered on too much sealer and its other signs, and uneven spots across a whole surface, rather than one edge, are covered on why a patio looks blotchy after sealing.

The third mechanism comes from below. ICPI, writing about concrete pavers, lists among the causes of a sealer whitening that "there is a source of efflorescence under the pavers (i.e, in the sand, base, or soil) moving through the joint sand and/or pavers," and names a poorly draining base among the causes of cloudiness. Because ICPI names the joint sand as one path, whitening that follows the joint pattern fits this mechanism, though ICPI names the pavers themselves as a path too.

Techniseal's data sheet for its Natural Look (NS) Protector, an acrylic film-forming sealer that also serves for joint stabilization, says any new concrete product can produce efflorescence and that the Protector, like any film-forming coating, "carries a small risk of trapping efflorescence beneath its film." The deposit itself is defined on the efflorescence glossary entry, and the general moisture causes behind a sealer turning white are on why sealed pavers turn white. A stain already on a paver before sealing is a different case again, under sealing over paver stains.