Mastic is a self-leveling elastomeric sealant that is used to seal the expansion joint between your coping and the concrete.
Due to chemical limitations, mastics tend to begin breaking down over time. It will become hard and brittle and will begin to pull away from the sides of the joint which breaks the watertight seal your pool needs.
Most companies have been using a product called Deck-O-Seal which in our experience bubbles and does not adhere properly. Dunrite Tile and Coping use a product called NovaLink SL pool deck joint sealant which we feel is the best product on the market.