Epoxy Shelf Life Helps Offset the Cost

Epoxy shelf life can last many years when resin and hardener are properly stored at room temperature and in closed containers to prevent contamination.

Those who have used polyester resins know that its shelf life is only about six months before it turns to a useless jelly-like substance. An abundance of expired material can easily cancel the initial cost savings of cheaper materials. Fortunately, WEST SYSTEM Epoxies offer an excellent shelf life. With proper storage and good housekeeping, you can always keep epoxy handy for whatever projects come along.

Store epoxy resin and hardener at room temperature. Keep containers closed to prevent contamination. With proper storage, resin and hardeners should remain usable for many years. After long storage, verify the metering accuracy of the pumps. Mix a small test batch to assure proper curing.

Over time, 105 Resin will thicken slightly and require extra care when mixing. Repeated freeze/thaw cycles during storage may cause crystallization of 105 Resin. Warm the resin to 125°F (by putting the can under a warming light or in a warm water bath) and stir to dissolve crystals, as you would with honey.

Hardeners, especially 205 Fast Hardener, may darken with age, but physical properties are not affected by color. If clear finishing, be aware of possible color shift if very old and new hardeners are used on the same project.

Epoxy Shelf Life, an Overview

Shelf life is the time epoxy can sit unused and still perform as designed. Epoxy shelf life commences when a material is made. We have mixed 15-year-old WEST SYSTEM 105 Epoxy resin with a newer hardener and it cured fine (See Shelf Life in Real Life). Although resin and hardener that is several years old should cure as designed, you may want to save it for applications where strength or clarity aren’t especially important.

The easiest, most accurate method to test questionable resin and hardener is to perform a simple pot life test. In an environment of 72°F, combine a total of 100 grams of resin and hardener (at proper ratio) in a 2″ diameter container. Begin timing and stir for exactly 2 minutes, then set it aside. Don’t hold it, because your body heat will accelerate the cure and skew the results. Take caution, as larger amounts of epoxy can produce high heat during cure. When the epoxy gels this is the end of the mixture’s pot life. If the resin and hardener are still good, they will cure within the “pot life” time indicated on the hardener container.

Note: Temperature has a dramatic effect on pot life. A temperature increase of 10°C (18°F), will cut the pot life approximately in half. A small change in the temperature where you are running the pot life test will affect the results.

Epoxy and pot life vs. temperature

WEST SYSTEM resins and hardeners do not have a strict shelf life. Certain changes may make them less effective for some jobs, but the resin and hardener will still be reactive. Because we are an ISO 9001 company, lot numbers are provided on containers and we keep retains of every batch we make up to 2 years from the date of manufacturing. However, the actual shelf life is typically far longer.

Changes in product characteristics


The viscosity of 105 Resin is very responsive to temperature. A seemingly slight decrease in ambient temperature can affect viscosity noticeably. This makes pumping, pouring, and stirring much more difficult. Simply warming the resin solves this problem.

Unrelated to temperature, the resin may thicken after several years. This condition is not reversible, but the product can still be used if extra care is taken to ensure thorough mixing with hardener.

Decades ago, the resin was susceptible to crystallization which made it appear milky. This was aggravated by exposure to freeze/thaw cycles. But crystallization was easy to revert by heating the resin, much the same way crystallized honey is restored. Today milky, crystallized epoxy is rare, thanks to a slight reformulation of the product. But if it crystallizes, warming the can in a water bath and gently stirring should resolve this.

WEST SYSTEM 205 Fast Hardener

This hardener may emit a strong ammonia-like odor if it has been stored in a closed container for a long time. Once opened, the ammonia odor escapes and the hardener’s scent returns to normal.

Exposure to metal may cause the 205 Fast Hardener to turn deep red or purple. The metal from certain large capacity dispensing pumps may accelerate this condition, and exposure to the metal of the hardener container may also cause this change. The hardener’s color will not affect the epoxy’s cured physical properties. In thin-film coatings, the color is difficult to detect, but mixed with white fillers it becomes very noticeable. The hardener color change is not reversible.

206 Slow Hardener

This product can also emit a strong ammonia-like odor but is not as likely to turn red.

207 Special Clear Hardener

Thin layers of 207 Hardener, (such as a drip from a mini pump) may solidify from exposure to air. This can plug the pump nozzle and have to be ‘picked’ away.

209 Extra Slow Hardener

A thin film of this hardener may also solidify when exposed to the air. This white solid is the result of a chemical reaction between a component of the amine hardener, moisture, and carbon dioxide (CO2) in the air. A strong ammonia-like odor may develop if it is stored in a sealed container for a long time.

When Contamination is the Issue

Most often, problems with older epoxy are not related to age but to some other condition, such as the presence of a contaminate.

Moisture contamination

Moisture contamination is a common culprit, especially when the product is left outside with the container top off and dew or rain finds its way into the container. Condensation inside a container can occur when stored in a cold environment and warmed quickly.

Hardener/resin cross-contamination

Sometimes through carelessness, hardener finds its way into the resin container or vice versa. This form of contamination is marked by chunks of cured material or stringy areas. This can happen if one spoon is used to draw both resin and hardener. Not much can be done to resolve this form of contamination, and both resin and hardener may be wasted.

Other contaminates

Other contaminants may factor in. An epoxy container stored on a damp surface such as a basement floor can corrode, contaminating the contents of the can. Other common epoxy contaminants include dirt, sawdust, sand or dust particles, all of which can easily enter an uncovered container. Contact with chemicals, oil, paint or solvents can also permanently damage resin or hardener.

—Excerpted from Epoxy Shelf Life by Captain James R. Watson, Epoxyworks 2