Not only in case of rain on new curb, (as in this case), but in ALL cases, STAMPED concrete, no matter what company installs it, needs to be resealed a couple of weeks after installation. It MUST be resealed before the first winter!!!
The video below shows the dramatic effect of resealing curb that has only the first coat.
We are using a sprayer, but a BRUSH works just as well or better.
Curb needs more protection than we can give it with just one coat done on the day of installation. In most cases, resealing is the homeowner's responsibility. This cuts down on the cost and it is the homeowner's first experience of a chore that needs doing every couple or three years after that.
If you don't want to reseal, then simply order an unstamped curb.
We have found after 18 years of curbing, that it is best if you experience the use of sealant while the curb is still new!
For full instructions, see this page on SEALANT