Two-Part Epoxy Mixing and Curing Guide

Mix ratio is not optional

Two-part epoxies cure by a chemical reaction between a resin and a hardener. The mix ratio is fixed by the formulation: too much resin or too much hardener leaves unreacted material and a weak, sometimes sticky bond. Measure by the method the product specifies — by weight or by volume — and do not guess.

Mix thoroughly, scraping the sides and bottom of the container. Unmixed streaks are uncured resin or hardener, and they become weak spots in the bond. For cartridges with a mixing nozzle, discard the first short length of extruded material, since it may not be fully mixed.

Working time and temperature

Every two-part epoxy has a working time: the period during which it can still be applied and the parts positioned. Working time shortens as temperature rises and as the mixed quantity grows, because the curing reaction generates heat. Mix only as much as you can use within the working time.

Cold slows curing. A joint assembled in a cold workshop may need much longer before it can be handled or loaded. Warm the parts and the adhesive to the recommended range where possible, and do not load the joint before the stated handling or full cure time.

Surface preparation for epoxy

Epoxy bonds best to clean, roughened surfaces. Remove oil, grease, rust and loose paint, and abrade smooth metals and plastics to give the adhesive something to grip. Some plastics bond poorly to epoxy regardless of preparation; confirm the plastic type before relying on an epoxy bond.

Gap filling is a strength of epoxy putties, which can fill voids and rebuild worn areas. But a thick putty section generates heat as it cures and may shrink or crack if applied far beyond the recommended thickness. Build up thick repairs in layers where the product guidance suggests it.

Common mistakes

The most common epoxy failures are wrong ratio, poor mixing, dirty surfaces and loading the joint too early. All four are preventable. A second common mistake is choosing an epoxy for a flexible joint: cured epoxy is rigid, so a joint that must flex needs a flexible sealant instead.

Finally, keep the two components separate until mixing, label opened containers, and store them as the product guidance states. Contaminated hardener or resin can ruin the next mix.

Choosing the right epoxy type

Epoxies come in many forms: fast five-minute grades for quick repairs, standard grades for general bonding, slow grades for large assemblies that need positioning time, and putties for filling and rebuilding. The right type depends on the working time you need, the gap you must fill and the loads the joint will carry.

Fast grades trade working time for speed and often for strength and heat resistance. Slow grades give more positioning time and usually a stronger, more complete cure. For a large or complex assembly, a slow grade is usually the safer choice even though it delays handling.

Bonding different materials

Epoxy bonds metals, concrete, stone, wood and many composites well. It bonds some plastics well and others poorly: polyethylene, polypropylene and PTFE are famously difficult, and some published descriptions explicitly list them as unsuitable. Surface treatment — abrasion, flame or plasma treatment, or a primer — can improve plastic bonding, but the result must be tested.

Dissimilar materials add another problem: different thermal expansion. A metal-to-plastic epoxy joint that is fine at room temperature may crack when heated or cooled, because the two sides expand differently and the rigid epoxy cannot absorb the difference. For such joints, a flexible adhesive may be the better choice.

Health, safety and cleanup

Uncured epoxy resin and hardener can cause skin sensitisation, and repeated contact can lead to allergy. Wear gloves, avoid skin contact, and wash promptly with soap and water — not solvent, which drives the material into the skin. Work in a ventilated area, especially with large quantities.

Clean tools before the epoxy cures. Once cured, epoxy is removed mechanically, not by solvent. Dispose of mixed waste as the local rules require, and never pour uncured resin or hardener into drains.

Testing an epoxy bond

Before a production run, test the bond on the actual materials, prepared as they will be in production, and cured under the actual conditions. Test to failure, not just to handling strength, and check the failure mode: a bond that fails in the substrate is stronger than the substrate, while a bond that peels cleanly off one surface is an adhesion failure.

Test at the service temperature as well as room temperature. An epoxy that holds at room temperature may soften or become brittle at the extremes of its service range.

Epoxy for concrete and stone repair

Epoxy is widely used to repair concrete cracks, anchor bolts and bond stone. For crack injection, a low viscosity grade penetrates fine cracks; for surface repair, a putty grade fills and shapes. The repair method and the grade must match: an injection grade used as a surface filler sags, and a putty grade cannot penetrate a fine crack.

Concrete must be sound, clean and dry enough for the grade used. Dust, laitance and moisture all weaken the bond. Some grades tolerate damp concrete and some do not — confirm the grade's published description for damp substrates before starting a below-grade repair.

Anchoring with epoxy needs the right hole size, cleaning and cure time. Dust left in a drilled hole is the most common cause of anchor failure. Brush and blow the hole, inject from the bottom up, and do not load the anchor before the stated cure time.

Epoxy coatings and flooring

Epoxy floor coatings protect concrete from chemicals, abrasion and dusting. They need a prepared floor — mechanically abraded, clean and dry — and the right primer for the concrete condition. A coating applied to a dusty or damp floor peels, whatever the product quality.

Chemical resistance belongs to the exact coating system, not to epoxy in general. A floor coating that resists oils may fail under acids or solvents. State the chemicals the floor will meet when asking for a system recommendation.

Temperature during application and curing affects flow, adhesion and cure. Cold floors slow curing and can leave a soft coating; hot floors shorten working time and cause bubbles. Plan coating work around the temperature range the system specifies.

A note on product selection

Guides on this site describe product types in general terms. They do not promise that any particular product suits a particular job. Before ordering, match the product code, the manufacturer's published description and your application's requirements, and confirm anything not stated in writing with the supplier.