Many two-part epoxies can be mixed by hand in small batches. A hardener is typically added to the epoxy resin, then the two are stirred with a utensil in a mixing vessel. Complete mixing may take several minutes.
Pot life varies from minutes to hours, and mixtures may require additional mixing after a period of time if they begin to settle.
With hand mixing, air bubbles become trapped in the epoxy and may remain visible on the substrate after the epoxy has cured, compromising the aesthetic appearance of the application. In fact, air bubbles are one of the most common challenges with two-part epoxy products.
Allowing the mixture to settle briefly can bring some bubbles to the surface, where they can be released. Once the epoxy is poured, applying heat can also release air bubbles before curing. However, epoxy is often flammable — so always follow safety guidelines for each product.
In addition, degassing the epoxy in its mixing vessel before application could resolve air entrapment. Some epoxy products smooth out when a moderate vacuum is pulled for several minutes. Substantial headspace in the mixing vessel is required because the air bubbles will rise up considerably before releasing.
For casting, a pressure pot may be recommended to reduce air entrapment. In this case, the epoxy is poured into the mold prior to pressurizing. This process also reduces the cure time.
Using premixed dual-chamber cartridges can reduce air entrapment in two-part epoxy. The dispensers also ensure the correct ratio of epoxy resin to hardener and the correct amount of stirring to combine the parts.
Preparing the substrate before the application of the epoxy can reduce the incidence of air bubbles while also ensuring optimal adhesion.
For example, when applying two-part epoxy over wood, preheating the substrate will cause the wood to expand and outgas initially. Then as the wood cools, it will contract and draw in the epoxy as it is applied, resulting in few or no air bubbles after curing.
Water or a solvent can be used to prepare some surfaces, according to manufacturer recommendations. However, solvents must be completely dry prior to applying the epoxy to avoid air bubbles caused by the solvent’s degassing. Simple abrasion with sandpaper can also ensure a smoother surface, but some plastics, for example, may require flame or corona treating to improve surface wettability.
Testing a number of sample mixes is recommended to determine the optimal process to reduce air bubbles in two-part epoxy.