Epoxy first became a household word in the 1960s, when a new breed of super-strong adhesives started to appear in local hardware stores. Its extensive use dates back as far as the 1940s. Like plywood, Plexiglas, and many other innovations that later became construction materials, epoxy technology was perfected during World War II, in this case as an alternative to metal fasteners for joining members in aircraft production. Briefly, epoxies are petroleum-based resins that cure to a solid state when combined with the right amount of the appropriate hardener. Epoxies are two-part systems—typically cans or tubes labeled A and B—that must always be mixed immediately before application. In contrast, a one-part system like yellow carpenters’ glue is usable right out of the bottle at any time.
Epoxies are also thermosetting—that is, they create heat as they cure. This is important to remember because substantial quantities of mixed resin and hardener—excess left in a mixing container, for example, or a pouring 1″ or more thick—can generate enough heat to melt the container or burn the epoxy. For the same reason, resin that is warm or used in a warm environment will cure at an accelerated rate. (Direct sunlight also will speed up the curing reaction.) The good news is temperature can be used to improve the working properties of the epoxy during adverse conditions, by cooling the resin in an ice bath on hot days to increase the pot life, or circulating warm air around a project in cold weather.