Air Dry is used when to turn your unwaxed resin in to waxed for your final top coat. If you only need one jug of resin or gelcoat and you plan to do multiple layers, then you need this as an additive for that last layer. It contains parrifin wax suspended in styrene solvent. The wax rises to the surface as the material cures and seals the surface from oxygen. Resin will only cure in the absence of oxygen so this is needed for the top layer. If it is not used the top layer will feel sticky (especially in the heat).