Evidence grade: A1 — First-hand testing Effort: N/A — reference, no procedure
Nothing dries. The moment resin meets hardener, the molecules begin linking into one continuous three-dimensional network — a chemical reaction called crosslinking. No solvent evaporates, no water leaves, nothing escapes into the air. Epoxy doesn't dry; it transforms. This one correction dissolves two of the most common confusions in repair work: "why is it still soft in winter" (the reaction rate depends on temperature) and "it feels dry, so it's ready" (tack-free is nowhere near full strength).
The mechanism
Two liquids become one plastic. An epoxy system is always two parts: the resin, carrying reactive epoxy groups, and the hardener, carrying the molecules that open and link those groups. Stirring them together starts polymerization — each molecule reaching out and bonding to its neighbors until the whole mass is one giant crosslinked network. Because the change is chemical rather than evaporative, ambient humidity barely matters; what matters is temperature, time, and the exact ratio.
The cure has distinct stages. The industry describes them the same way across manufacturers:
- Liquid — open time. After mixing, the blend stays fluid and workable. All assembly, clamping, and shaping must happen here.
- Gel — initial cure (the "green stage"). The mixture begins to gel, or "kick off": it's no longer workable, passing from a tacky gel to the firmness of hard rubber — dentable with a thumbnail. It becomes tack-free about midway through this phase. While still tacky, a fresh coat will still chemically bond to it without special preparation.
- Solid — final cure. The epoxy is a hard solid that can be dry-sanded; you can no longer dent it with a thumbnail. Clamps can come off. But the reaction isn't finished: the mixture keeps curing for days to two weeks at room temperature, gradually becoming a fully inert plastic solid.
The published numbers show how long this arc really is. A typical marine epoxy (105 resin / 205 fast hardener) reaches a solid state in about 5–7 hours but doesn't reach maximum strength for 5–7 days. Even a five-minute adhesive reaches initial cure in 3–4 hours and workable cure in 7–10 hours — the "five minutes" is only the pot life, the time the mixed batch stays usable.
Wrong ratio = never cures. Full stop. A skewed mix doesn't cure slowly or weakly — the unreacted excess has nothing to react with, so the batch stays soft, gummy, or tacky indefinitely. It cannot be "dried out," heated into curing, or rescued by adding more of the missing part afterward. That batch is scrap; the only fix is removal and a fresh, correctly measured mix. Most cure failures trace back to exactly this.
Why this dissolves the two classic confusions
"It's slow in winter." The reaction rate is temperature-dependent: warmer epoxy cures faster, cooler epoxy cures slower — the same chemistry, a different pace. Winter doesn't pause evaporation (there is none); it slows the crosslinking reaction itself. The remedy is warmth — warming the resin, the hardener, or the workpiece — never extra hardener.
"It feels dry, so it can take a load." Tack-free happens midway through the initial cure — roughly the first third of the journey. A thumbnail-dentable surface is still weeks of chemistry away from full mechanical properties. The manual's own test is physical, not temporal: when you can no longer dent it with a thumbnail and it dry-sands cleanly, it's at most of its ultimate strength — and the remaining cure still takes days.
Honest limits
- Cure schedules are temperature- and mass-dependent. A thick mass self-heats and cures faster than a thin film (see Repair Lab B8); a cold shop slows everything. Published times are measured under controlled conditions, not yours.
- "Hard to the touch" is not "fully cured." Full mechanical properties — strength, heat resistance, chemical resistance — require the complete cure cycle, measured in days, not the hour the surface stopped being tacky.
- This describes two-part thermoset epoxies. One-part "epoxy" adhesives, UV-cure resins, and moisture-cure products follow different chemistries; nothing here transfers to them automatically.
- Post-curing with modest heat after the solid stage can improve thermal performance — but that's a deliberate process with its own guidance, not a shortcut around the waiting.
Sources
- Gougeon Brothers, Inc. — WEST SYSTEM User Manual & Product Guide (Aug 2024). Cure stages: "Liquid — Open Time" / "Gel — Initial Cure Phase" / "Solid — Final Cure Phase"; "The mixture will become tack free about midway through the initial cure phase"; "The mixture will continue to cure for the next several days to two weeks at room temperature, becoming an inert plastic solid"; "DO NOT attempt to adjust the epoxy cure time by altering the mix ratio"; "Most problems related to curing of the epoxy can be traced to the wrong ratio of resin and hardener." URL: https://cdn.shopify.com/s/files/1/0634/5012/0257/files/2024.08_WS_User-Manual_web_low_res.pdf?v=1757064964
- Wessex Resins — Typical Physical Properties of WEST SYSTEM Epoxy (Version B, 08/08). Cure-to-solid times (5–7 hours for 105/205) and cure-to-max-strength times (5–7 days). URL: https://www.ecfibreglasssupplies.co.uk/user/TechnicalDataSheet/618.pdf
A two-part epoxy in a pre-metered 1:1 cartridge runs the same crosslinking chemistry described here — the ratio the reaction demands is handled before you ever pick it up.
Repair Lab · Release 1 · Evidence last checked September 29, 2026 · A project of AOJEL