You mixed the batch, spread the coating, and came back the next morning to a surface that should be hard. Instead it is tacky to the touch, soft enough to dent with a fingernail, or stubbornly gummy days past the cure time on the data sheet. Sometimes the whole part is soft; sometimes only a patch, or just a greasy film on top. An under-cured epoxy is one of the most common and most costly failures in amine-cured systems, and the cause is almost always one of nine things you can diagnose in a few minutes.
The short version: Epoxy that stays tacky, soft, or sticky is under-cured, meaning the resin and hardener never fully reacted into a crosslinked network. The usual culprits, in rough order of how often they bite, are an off mix ratio, incomplete mixing, low temperature, and a greasy amine-blush film on the surface. Behind those sit thin films that cure slowly, old or moisture-contaminated hardener, reusing one hardener’s ratio for another (the wrong AHEW), surface contamination, and simply not giving the system enough cure time. The honest part: a slow or cold cure can often be rescued with heat and time, but an off-ratio or badly mixed batch frequently cannot be fully rescued and has to be removed and redone. Adding more hardener never speeds the cure; it guarantees the opposite.
Why amine-cured epoxy under-cures
Amine-cured epoxy is not a base plus a catalyst. It is two co-reactants: each epoxide ring on the resin reacts with one active amine hydrogen (N-H) on the hardener, and those bonds build the three-dimensional network you feel as hardness and chemical resistance. The reaction is stoichiometric, so anything that disturbs the ratio of epoxide to amine hydrogen, slows the reaction down, or contaminates the interface leaves reactive groups unreacted. Unreacted groups are exactly what tacky, soft, and sticky feel like. That single mechanism explains every cause below, which is why the fix always traces back to ratio, mixing, temperature, or the surface.
Diagnose it: root cause, symptom, fix
Match what you are seeing to the table, then read the section it points to. The salvage column is the honest one: read it before you reach for a heat gun.
| Root cause | Symptom you see | Fix / salvageable? |
|---|---|---|
| Off mix ratio (too much or too little hardener) | Uniformly soft and sticky, dents or moves days later, never hardens | Often not fully salvageable. Remove and redo at the correct weight ratio; heat helps only if it was slightly off |
| Incomplete / under-mixing | Patchy, streaky soft and tacky spots next to hard areas | Scrape uncured material; remix the next batch the full time and scrape sides and bottom |
| Low temperature | Whole batch slow, gummy, or rubbery, tacky long past the stated time | Usually recoverable. Add heat, give more time; cure time roughly doubles per ~18 F drop |
| High humidity / amine blush | Greasy, hazy, tacky film on top, bulk cured underneath | Wash off with warm water and abrade before recoating (the amine-blush guide) |
| Thin film / small mass | Thin coat stays tacky while a thick batch nearby went hard | Add heat and time; thin films generate little exotherm and cure slower |
| Old or contaminated hardener | Slow cure, foaming, or a persistently soft film | Test a small batch first; replace hardener that has aged or taken on moisture |
| Wrong AHEW (reused another hardener’s ratio) | Looks like off-ratio: soft, under-crosslinked | Recompute parts per hundred from the actual AHEW and EEW (the selection guide) |
| Surface contamination (silicone, oil, release agent, water) | Fish-eyes, craters, localized tacky or uncured zones | Clean and prep the substrate; keep silicone away from epoxy work |
| Insufficient cure time / no post-cure | Firm but faintly tacky, short of full hardness and Tg | Allow the full cure; post-cure with heat to reach rated properties |
Get the mix ratio right: by weight, per AHEW and EEW
The single biggest cause of a soft, never-hardening batch is the ratio. Epoxy is not mixed by feel, by equal volumes, or by adding “a bit extra” hardener to push it along. Because each epoxide group needs one amine hydrogen, you match equivalents, and equivalents are a mass concept. The stoichiometric ratio in parts of hardener per hundred parts of resin by weight is phr = (AHEW / EEW) x 100, where EEW is the resin’s epoxy equivalent weight and AHEW is the hardener’s amine hydrogen equivalent weight (Huntsman formulation guide, PDF).
The trap is that AHEW varies enormously between hardeners, so a ratio that is correct for one is badly wrong for another. A standard liquid bisphenol-A resin cured with diethylenetriamine (DETA) takes about 11 parts hardener; the same resin with polyetheramine D-230 takes roughly 32. Reuse the DETA number on the polyetheramine and you have starved the system of amine, and the part stays soft. Always set the ratio from the actual EEW and AHEW on the certificates of analysis for the specific grades you bought. The full worked math, with a class-by-class comparison, is in our epoxy curing agent selection guide. One rule overrides every shortcut: more hardener does not cure faster. Off-stoichiometry leaves unreacted resin or amine in the film and you get a soft, sticky result either way (WEST System Epoxy Problem Solver).
Mix it completely
A correct ratio mixed badly fails the same way an off ratio does. Resin and hardener that are measured right but stirred for too short a time, or stirred without scraping the container, leave streaks and pockets of unreacted material that stay tacky next to fully hard zones. Mix for the full time the data sheet specifies, scrape the sides and bottom of the pot as you go, and for small batches use the double-cup method: mix, transfer to a clean container, and mix again. The resin clinging to the first cup’s wall is the part most likely to come out soft.
Cure warm and above the dew point
Temperature is the third big lever. Most ambient amine systems are formulated to cure near 20-25 C (68-77 F); below roughly 10-13 C (50-55 F) the reaction slows sharply, and a useful rule of thumb is that cure time roughly doubles for every 18 F (10 C) drop (WEST System Epoxy Problem Solver). A batch that looks gummy in a cold shop is frequently not ruined; it is slow, and gentle heat plus patience finishes it.
Humidity attacks the surface specifically. When free amine at the curing face meets atmospheric moisture and carbon dioxide, it forms a greasy, hazy, tacky carbamate film, amine blush, while the bulk underneath cures normally. The fix is not more heat; it is to wash the water-soluble blush off with warm water and abrade before recoating, covered in detail in our amine blush guide. A related failure is curing on a cold surface near the dew point: keep the substrate at least 5 F above the dew point so condensation does not form under the film and inhibit cure (Epoxyworks, cold-weather bonding). Thin films compound the cold problem, because a thin coat generates little exotherm to warm itself and tracks ambient temperature closely, so it cures slower than the same chemistry poured in mass.
Check the hardener and the surface
Two quieter causes round out the list. First, the hardener itself. Amine hardeners oxidize and can pick up moisture from the air once opened; a container that has aged past its shelf life or sat open can cure slowly, foam, or leave a soft film. If you suspect it, mix a small test batch and confirm it cures within the rated time before committing a production lot. Second, contamination at the interface. Silicone in particular, from a mold release, a nearby spray, or a contaminated tool, causes fish-eyes and localized uncured spots, and oils, waxes, or water on the substrate do the same. Clean and prep the surface, and keep silicone products well away from epoxy work areas.
What you can rescue, and what you can’t
This is where honesty saves money. A cure that is merely slow, because it was cold, thin, or under-heated, is usually recoverable: move the part somewhere warm, hold it there, and a marginal under-cure will often finish polymerizing. Many manufacturers specify an elevated-temperature post-cure to drive a firm-but-tacky part to full hardness and rated Tg, and the same heat can finish a stalled slow cure.
An off-ratio or badly mixed batch is a different situation. Once the stoichiometry is wrong, there is unreacted resin or amine permanently dispersed in the film, and heat cannot manufacture the reactive partner that was never there. A small ratio error may improve with post-cure, but a meaningfully off batch, or one with soft, movable areas days later, generally has to be scraped off and redone. The cheap move is to get the ratio and mixing right the first time; the expensive move is trying to bake a fundamentally off batch into spec.
Accelerators for cold or slow cure
If your problem is chronic slow cure rather than a one-off mistake, the fix may be a faster chemistry or an accelerator rather than more heat. A slow ambient amine such as isophorone diamine (IPDA) can be sped up with a tertiary-amine accelerator like DMP-30 (tris(dimethylaminomethyl)phenol) at a few parts per hundred, which lowers the activation energy and pulls in the room-temperature gel time. Imidazoles such as 1-methylimidazole (NMI) play a similar accelerating role, and research shows that pairing a tertiary amine with an imidazole reduces cure activation energy further than either alone (Journal of Applied Polymer Science, 2026). For a faster ambient base chemistry, m-xylylenediamine (MXDA) cures quicker than IPDA, and an unmodified aliphatic amine like triethylenetetramine (TETA) is among the fastest in warm, dry conditions. The trade-off is consistent across all of them: faster cure means shorter pot life and a hotter exotherm, so dose to the gel time and mass you actually need. Which accelerator and how much is laid out in the selection guide.
Buying epoxy curing agents
RawSource supplies the full amine-hardener range, with the AHEW and EEW data you need to set the ratio correctly: aliphatic amines (DETA, TETA), cycloaliphatic and araliphatic amines (IPDA, MXDA), polyetheramines, the DMP-30 accelerator, and imidazole curatives such as NMI, for coatings and industrial manufacturing formulators in drums, IBCs, and bulk with CoA documentation. Tell us your resin EEW, cure temperature and humidity, pot life, and hardness target, and request a sample to qualify cure on your own system.
Frequently asked questions
Why is my epoxy still sticky?
Sticky or tacky epoxy is under-cured: the resin and hardener did not fully react. The most common reasons are an incorrect mix ratio (including adding extra hardener, which never speeds cure), incomplete mixing that left unreacted streaks, or too low a temperature. A greasy, hazy tackiness only on the surface, with hard epoxy underneath, is usually amine blush rather than a failed cure. Identify which one you have before deciding whether to wait, heat, or redo it.
Can I fix uncured epoxy?
Sometimes. If the cure is merely slow because it was cold, thin, or under-heated, moving the part somewhere warm and allowing more time, or a specified elevated-temperature post-cure, will often finish it. If the batch is soft because the ratio was off or it was poorly mixed, heat cannot supply the reactive partner that was never there, so an off-ratio or badly mixed batch frequently cannot be fully rescued and must be removed and redone. Confirm cure on your own system before relying on a part.
Does epoxy cure in cold weather?
Most ambient amine systems slow sharply below about 50-55 F (10-13 C), and standard grades may not cure at all near freezing. Cure time roughly doubles for every 18 F (10 C) drop, so a cold batch is often slow rather than dead and can be finished with added heat and time. Keep the substrate at least 5 F above the dew point so condensation does not form under the film. For routine cold-shop work, choose a fast or low-temperature hardener, or add an accelerator, rather than fighting the temperature each time.
What is the correct epoxy mix ratio, and why does it matter so much?
The ratio is set by the chemistry, not by feel: each epoxide group needs one amine hydrogen, so you mix by weight using phr = (AHEW / EEW) x 100. Because AHEW differs widely between hardeners, the same resin can need roughly 11 parts of one hardener and 32 of another, so you cannot reuse a ratio across hardeners. Off-ratio mixing leaves unreacted resin or amine in the film and produces a soft, sticky, under-crosslinked part. Always use the actual EEW and AHEW from the certificates of analysis for your specific grades.
Will adding more hardener make epoxy cure faster or harder?
No. Unlike polyester resins, epoxy does not cure faster with more hardener; extra hardener pushes the system off stoichiometry and leaves unreacted amine in the film, giving a softer, stickier, weaker result with lower chemical resistance. If you need a faster cure, use a faster hardener or a small dose of an accelerator such as DMP-30 or an imidazole, and keep the resin-to-hardener ratio at the value the AHEW and EEW dictate.
Why did only part of my epoxy cure while the rest stayed soft?
Patchy curing, hard in places and soft in others, almost always points to incomplete mixing: unreacted resin or hardener clinging to the container wall or bottom never got blended in. Mix the full specified time, scrape the sides and bottom, and use the double-cup method on small batches. Localized soft spots or fish-eyes instead of streaks usually indicate surface contamination, such as silicone or oil, at those points.
Editorial note. This article is general technical guidance for coatings, adhesive, composite, casting, and industrial formulation professionals. Cure behavior, the correct mix ratio, salvageability, and post-cure response depend on your specific resin, hardener grade, ratio, film thickness or mass, and cure environment, and must be validated on your own system; the Certificate of Analysis governs the EEW, AHEW, and grade you buy. Amine hardeners are corrosive and can cause skin and eye burns and skin and respiratory sensitization; review the current Safety Data Sheet (SDS) and use appropriate PPE before handling, and follow the manufacturer’s cure and post-cure schedule. Products are sold for industrial and professional use only. Nothing here is a medical, health, or safety claim. RawSource makes no warranty, express or implied, and assumes no liability for use of this information.
