The problem is blush. Most epoxies have a fairly good amount of time for a chemical bond. (Like system 3) But the world has changed and now blush free epoxies are everywhere. That said, I still think its wise to add coats ASAP. I always wet out epoxy in the evening and first thing in the morning another coat went on and that late afternoon, another coat went on.

I finally settled in on RAKA blush free. However I used WEST, Mas, and System III and a few odd ball brands too and never had a problem. If for some reason I couldn't get to it for a while, I would just scrub the surface with an abrasive pad and water and wipe it off with paper towels.