I poured a beach scene in a silicone coaster mold and it came out beautiful, but the top is kind of concave. It’s been 3 days and I’d like to pour a doming layer on top…Question: Do I have to sand the top first? And if so, what grit of sandpaper should I use? I would like to present it as a gift on Christmas Day, so if someone could answer quickly, I would be forever grateful!