Welcome Johnny
Your bench sounds good. Yes the sand is best kept moist, to spread the warmth and prevent cables overheating. Water it gently until drips come out of the bottom, before switching on. Some drainage holes are needed at the bottom. Sometimes it is worth lifting trays and pots to check they are not drier underneath than they look on top, in which case water well, also water the sand if ever the electricity is on and no pots or trays are sitting on it.