An incorrect mixing ratio is the number one reason resin fails to cure, and unlike a cold shop or a humid day, most ratio errors cannot be rescued after the pour. Epoxy is a stoichiometric reaction: resin and hardener must meet in a fixed proportion so every molecule finds its partner and crosslinks. Get the proportion wrong and you are left with unreacted material that stays soft, gummy, brittle, or rubbery forever. The comparison table above maps each kind of ratio error to what you will see, the number that matters, whether the piece can be saved, and the exact recovery path. Read your symptom against that table first, then work through the matching cause below.
Here is the short version. Pick one measuring system and never blend them. Measure by weight on a 0.1-gram scale using the product’s published weight ratio, or measure by volume in graduated cups using its volume ratio. Mix for a full 3 to 4 minutes while scraping the sides and bottom, and never adjust the ratio to change cure speed. Do that and you eliminate the overwhelming majority of cure failures before they start.
Why Ratio Is Non-Negotiable
Two-part epoxy is not like baking, where a little extra of one ingredient is forgiven. The resin (Part A) and hardener (Part B) react in a set chemical proportion, and the cured polymer only reaches full hardness, strength, and chemical resistance when that proportion is correct. Manufacturers are blunt about it: WEST SYSTEM, for example, warns that most curing problems trace directly to the wrong ratio and that you should never alter the ratio to change cure time. A typical 50/50 epoxy carries a working tolerance of only about plus or minus 10 percent, and even inside that band the cured properties degrade. There is no surplus that simply sits inert; excess of either part stays unreacted and weakens the result.
Cause 1: Too Little Hardener (Resin-Rich)
If the entire pour is uniformly soft, gummy, and never sets, you almost certainly shorted the hardener. With too little Part B, a large fraction of the resin has no partner to react with, so it stays liquid or tacky indefinitely. This is the failure most people misread as a slow cure. It is not slow; it is stalled. Waiting longer, warming the room, or pouring a fresh coat on top will not save it, because the chemistry underneath is permanently incomplete.
The only fix is removal. Scrape off the whole pour, wipe residue with 91 percent or higher isopropyl alcohol, and re-pour a freshly measured batch. To avoid a repeat, measure by weight on a 0.1 g scale like the American Weigh LB-3000 in the comparison above, which stabilizes in 3-5 seconds and is precise far beyond epoxy’s needs.
Cause 2: Too Much Hardener (Hardener-Rich)
Excess hardener is the opposite mistake and just as damaging. Counterintuitively, more hardener does not cure the epoxy harder; it leaves unreacted amine in the mix that can make the final piece bendy and rubbery. It also slashes pot life, so the batch can flash-cure in minutes, generate excessive heat, and in extreme cases smoke. Resin Obsession’s guidance is direct: if you catch an over-hardened mix before pouring, do not use it, and move it somewhere it cannot start a fire.
There is no save-it-with-more-resin trick. Adding Part A to an already-mixed hardener-rich batch does not rebalance the chemistry. Remove the failed material and re-pour at the exact ratio. If you needed a faster cure, that is what a fast hardener or a warmer room is for, never extra Part B.
Cause 3: The Weight-vs-Volume Trap
This is the error that catches the most careful beginners, because they did measure, just in the wrong units. Resin and hardener have different densities, so a product’s volume ratio is almost never its weight ratio. ArtResin is 1:1 by volume but roughly 100:84 by weight; TotalBoat High Performance is 2:1 by volume but 100:45 by weight. If you put a 1:1-by-volume product on a scale and weigh it 1:1, you have under-dosed the hardener and the pour will stay soft, exactly like Cause 1.
The fix is discipline, not skill. Choose your system and follow the matching number. Weighing? Use the product’s stated weight ratio (100:84 for ArtResin, 100:45 for TotalBoat High Performance). Measuring by volume? Use graduated cups like the Let’s Resin silicone set and the volume ratio printed on the kit. Never read a volume cup as if it were a weight ratio. Our breakdown of how cure chemistry interacts with timing is in resin working, cure, and demold times.
Cause 4: Slight Off-Ratio Within Tolerance
Not every ratio error produces a puddle. A mix that is off by a few percent often does cure, but the cured piece is quietly compromised: softer, more brittle, and less resistant to heat, chemicals, and moisture than a correctly mixed pour. A 1:1 epoxy generally tolerates around plus or minus 10 percent before this becomes obvious, and tighter ratios like 100:45 are less forgiving still. The danger here is that the failure is invisible until the piece dents, yellows, or cracks under stress months later.
If a slightly off pour cured firm and the application is cosmetic, you can sometimes sand for tooth and recoat with a correctly mixed top layer. If the piece is structural or load-bearing, do not gamble on degraded properties; redo it. Choosing the right hardener speed for your conditions, covered in slow cure vs fast cure epoxy, removes the temptation to fudge the ratio for timing.
Cause 5: Under-Mixing Creates a Local Off-Ratio
Even a perfectly measured batch fails if it is not mixed thoroughly, because unmixed resin and hardener cling to the walls and bottom of the cup. That clinging material is effectively off-ratio, and it shows up as sticky streaks or soft patches concentrated near the edges. Mix vigorously for a full 3 to 4 minutes, deliberately scraping the sides, the base, and the stir stick itself. ArtResin specifies at least 3 minutes of mixing with continuous scraping for this exact reason.
The reliable defense is the two-cup method: mix thoroughly in the first cup, pour into a clean second cup leaving the scraped residue behind, then mix again. The Let’s Resin cups are built for it, with bold ratio markings and non-stick walls. Because under-mixing usually fails only at the surface or edges, sanding the tacky layer and recoating with a fully mixed batch typically rescues the piece.
Cause 6: Adjusting the Ratio to Change Cure Speed
A specific and avoidable version of the over-hardener mistake deserves its own warning: never add extra hardener to make epoxy set faster. Cure speed is governed by the hardener grade and the ambient temperature, not by the ratio. TotalBoat’s High Performance line illustrates this cleanly: the fast hardener gels in about 10 minutes, medium in 25, and slow in 40, all at the same 100:45 weight ratio. The speed lives in the chemistry of the hardener, not in dosing more of it. If you need a faster cure, buy a fast hardener or warm the room to 75-85F. If you need more working time, use a slow hardener or cool the room. Touching the ratio only buys you a soft, brittle, or overheated result.
Recovery Decision: Remove or Recoat
Once you have identified the error, the recovery path is binary, and the deciding question is where the failure lives. If the pour is soft, gummy, or rubbery throughout, the chemistry itself is wrong and no surface treatment will fix it: scrape the whole batch out, wipe with 91 percent or higher isopropyl alcohol, and re-pour a correctly measured, fully mixed batch. If the failure is only a thin tacky skin from under-mixing or a slightly off top layer, treat it as a surface problem: sand lightly (around 220 grit) for tooth, remove the dust, and recoat with a fresh, correctly measured batch. When in doubt, press a fingernail in; if it sinks into soft caramel through the body of the pour, it is a remove-and-redo, not a recoat. For the broader symptom map of tacky and soft cures beyond ratio errors, see sticky uncured epoxy resin fix, and if bubbles are tangled up with your cure issues, epoxy resin bubbles: how to fix covers that.
Prevent the Next Off-Ratio Batch
Prevention comes down to four habits. First, measure by weight on a 0.1 g scale whenever you can, using the product’s published weight ratio, because weight is immune to the density trap that volume measuring hides. Second, if you measure by volume, use clearly graduated cups and the volume ratio, and never cross-read the two systems. Third, mix 3 to 4 full minutes with the two-cup method so a good ratio is not wrecked by clinging unmixed material. Fourth, never touch the ratio to change cure speed; reach for a different hardener grade or adjust temperature instead. Browse the full troubleshooting library at the resin troubleshooting hub or the troubleshooting index for the rest of the common resin failures.
The bottom line: match your symptom to the table, decide remove-versus-recoat by whether the failure runs through the whole pour or sits on the surface, and prevent the next one with a scale, the correct ratio for your measuring system, a full mix, and a hardener chosen for speed instead of a ratio tweaked for it.