Yes, we would recommend you seal the flowers before adding them to the casting mold. Sealing the flowers will help to prevent air bubbles from forming, and it will allow the flower to better set in the right position in the mold. This is done by simply dipping the flowers in the resin first to form a very thin coating around them. When you add the flowers into the mold, they will already have a resin seal and will end up setting much better.