How long do epoxies take to cure?

With two pack epoxies, where the curing mechanism is an internal chemical reaction, full cure could take months. During the curing process, there are many chemicals reacting with each other to form a highly cross linked network, turning liquid into solid. This slowly makes it harder for any remaining non-cross linked parts to react. The temperature and humidity during the curing process plays an important role in the speed of cure, as warmth provides the system with more energy to react. So this means the curing process is faster in warm temperatures, and slower when it’s cold.

High humidity conditions can affect epoxy materials by causing a surface reaction that leads to a sticky substance being formed often mistaken as a lack of cure. This is more often observed with solvent free epoxy systems. See article on Blushing/blooming of epoxy materials.

High humidity levels can affect solvent based products by slowing down the evaporation of solvents out of the system which in turn slows down the curing rate.

In reality, full cure is usually defined as the time it takes for the product to meet all of its required end use properties. This is usually around 7 – 30 days depending on the temperature during cure.

Epoxy based products may appear to be cured ie they are dry on the surface, but they may not have reacted fully throughout the paint film.

Therefore it is important that the recommended over coating times are followed to avoid a partially cured product from absorbing solvent from the subsequently applied coatings leading to potential problems such as but not limited to blistering or poor adhesion.

Datasheets for epoxy based products usually state a minimum temperature of cure. Be sure to follow them very closely. If curing is attempted below these temperatures, the product may not cure at all until the temperature rises, extending the whole process.