Resorcinol glue, also known as resorcinol-formaldahyde, is an adhesive combination of resin and hardener that withstands long-term water immersion and has high resistance to ultraviolet light. The adhesive, introduced in 1943, has been popular in aircraft and boat construction.
Until the invention of epoxy resin, resorcinol was one of the most common marine glues. Unlike epoxy, it does not have gap filling properties, requiring joints to be close fitting and clamped under pressure to achieve good results. The glue comes in two parts — a red syrup and a light brown powder that were mixed to form glue. Uncured resorcinol has a relatively short shelf life of about two to three years, depending on storage temperature. Its use has declined since the 1990s due to the ease of use and versatility of epoxy glues and fillers.
Although the greater ease an versatility of epoxy makes it much more popular, epoxy has poor UV resistance and in most structural uses only a modest heat resistance, making it less than ideal many outdoor uses. Resorcinol remains the most suitable adhesive for exterior and marine use. Unlike epoxy, it is not gap-filling, so requires a higher standard of workmanship and joint fitting.