Evidence grade: A1 — First-hand testing (TDS/spec sheets, ASTM/ISO, lab tests)
Answer: A mix ratio is chemistry, not a recipe preference: the hardener must chemically match the resin molecule-for-molecule. Go off-ratio and the unmatched half never crosslinks — the batch stays soft or sticky permanently, and no amount of waiting fixes it. Scrape it off and start over [1][2].
Effort: N/A — debunk/reference, no procedure
Why the numbers differ
Epoxy cures when the hardener's reactive sites link up with the resin's reactive sites (see Repair Lab B1). Different hardeners carry different numbers of reactive sites per gram, so each formulation has its own stoichiometric ratio:
- West System 105 resin takes 5:1 with 205/206 hardeners, but 3.5:1 with 207/209 — same resin, different hardener chemistry, different ratio [1].
- West System Six10 is 2:1 by volume, 2.25:1 by weight — the two columns differ because resin and hardener have different densities (1.17 vs 1.04 g/cm³) [2].
- Other products use 1:1 or 1.2:1 for the same reason: their hardeners are chemically different.
Two consequences. First, volume and weight ratios are not interchangeable — always use the column that matches how you're measuring. Second, ratios are not comparable across brands as a quality signal. A 5:1 system isn't "stronger" or "weaker" than a 1:1 system; the number just describes that product's chemistry.
What actually happens when you're off
The cure is a matching game. Too little hardener: resin molecules with nothing to link to stay unlinked — the result cures soft, flexible, and weak. Too much hardener: the excess has nothing to react with and stays behind as a plasticizing contaminant — often the surface stays tacky indefinitely [2].
The most destructive myth is "extra hardener makes it cure faster." It doesn't. The reaction speed is set by temperature and the formulation (see Repair Lab B8); extra hardener just guarantees unreacted hardener in the finished part. Under-mixing has the same effect locally: streaks of off-ratio material that never harden.
The rule that matters most: an off-ratio or badly mixed batch is scrap. There is no rescue — not heat, not time, not a second coat on top. Remove it mechanically and remix. This is the single most useful "stop rule" in epoxy work, and it belongs to a reference article because it is chemistry, not technique.
Why cartridges exist
Six10 ships in a self-metering cartridge whose molded chambers and piston "accurately control the ratio and prevent the resin and hardener from contacting each other until they are outside the cartridge" [2]. The entire product design is an admission: humans are bad at ratios, so the ratio was engineered out of human hands. If you mix from bulk cans, use pumps or a scale — eyeballing is how off-ratio batches happen.
Honest limits
- "Off-ratio never cures" has a tolerance band: manufacturers design for small metering errors, and a few percent off still cures acceptably (exact tolerance bands were not published in the cited sheets — unverified — which is itself the point: don't test the edge).
- Heating speeds the reaction (B8) but cannot complete a stoichiometrically impossible cure — heat plus wrong ratio gives you a faster bad result.
- This article covers two-part epoxies. One-part heat-cure epoxies and UV-cure systems have no mix ratio at all — different chemistry, different rules.
Sources
- West System — Typical Physical Properties of West System Epoxy (Wessex Resins, Version B 08/08). Mix ratio by weight: 5:1 (105/205, 105/206), 3.5:1 (105/207, 105/209), 1.2:1 (G/flex). URL: https://www.ecfibreglasssupplies.co.uk/user/TechnicalDataSheet/618.pdf
- West System — Six10 Thickened Epoxy Adhesive TDS (Rev. 3). Mix ratio 2 parts resin : 1 part hardener by volume, 2.25:1 by weight; self-metering cartridge with molded chambers that "accurately control the ratio." URL: https://eu.westsystem.com/app/uploads/2023/07/Six10Thickened-Epoxy-Adhesive-Rev-3.pdf
With a pre-metered 1:1 cartridge, metering is mechanical — so the ratio failure mode is a worn or uneven cartridge, not a measuring mistake.
Repair Lab · Release 1 · Evidence last checked September 30, 2026 · A project of AOJEL