What happens if it rains right after sealcoating?

What the rain is doing

Sealer cures by losing water. Before that happens the film is still water soluble, so rain does not sit on top of it, it takes some of it away. On a sloped drive the runoff carries it to the bottom, which is why the damage shows up as streaks running downhill and a dark line where it pooled.

What it looks like the next morning

Pale streaks, blotches where the coat is thin, and sometimes bare gray asphalt showing through. It is obvious, and it does not improve. Sealer does not darken back up once the material is gone.

Dew and humidity count too

Rain is the obvious version. A heavy dew overnight on a coat put down late in the afternoon does a milder version of the same thing, and so does a cool humid day where the water never really leaves.

That is why a coat that goes down in the morning is in better shape than one that goes down at five, and why the last hour of daylight is not a good hour to start a driveway.

How it gets handled

The washed areas get cleaned and recoated once the surface is dry again. It is not a full redo on most jobs, because the parts that were sheltered or that had longer to set are usually fine.

The way to avoid it is the forecast, and it is worth saying that a contractor who moves your date because of the radar is doing the right thing. A coat put down ahead of rain is a coat you pay for twice.