Sealants are categorized as hardening and nonhardening. Whether a sealant is hardening or nonhardening depends upon its chemical composition and curing characteristics rather than its initial form. Sealants generally come in nonsolid forms in a wide range of viscosities. Some epoxy sealers come in powdered form and must be melted to be applied. Certain asphalt-based sealers and waxes are solid and applied by a hot-melt system. Thermosetting film adhesives used for sealing also come in tape form and require heat and pressure for curing.

Hardening sealants may be rigid or flexible depending on their composition. Nonhardening types are characterized by plasticizers that come to the surface continually, so the sealant stays "wet" after application. Nonhardening sealants are characterized by the "mastics" that are applied to seams with a trowel or brush. They cannot be depended on for joining, although some formulations are used as adhesives in very low-stress joints.

Tapes are available in a variety of backings and with pressure-sensitive or solvent-activated adhesives. Some nonhardening sealants are so viscous they can be packaged in "tape" form. For purposes of this chapter, they are not considered to be sealing tapes but simply another form of a heavy-consistency nonhardening sealant.