Hi and thanks for using JustAnswer.com. The hydraulic cement is quick setting. After 30 hours you will have no issues of attempting to bond to uncured cement. However all cement is conductive of water. I would dry the surface of the patch and surrounding cement by briefly heating/waving with a torch and immediatly applying the sealer before moisture returns to the surface.
I hope that this information was helpful to you. If it was please remember to click "ACCEPT" on your screen to make sure that I am paid for my efforts. By clicking ACCEPT you are not giving up your ability to ask more questions pertaining to this subject, and I will be happy to respond to these as well. Please take a moment and leave feedback, it is very important!
General C&ontractor
35 years of troubleshooting construction, remodel, component & material failures. What to do next