Shop epoxy answers to heat and batch size — not to your stopwatch….. A cup that looks fine at mix time can kick while you're still tooling the second fillet. Batch size, shop temp, and the early warning signs of a kick — that's today's tip.
Why it matters: On a Chamberlain Marblehead gunning dory (or any stitch-and-glue / cold-molded hull), you're constantly mixing small epoxy batches for scarfs, fillets, and wet-out. Mix too big and the pot cooks itself before the work is done. Mix in a hot shop with Fast hardener and you get the same result faster. WEST SYSTEM / Gougeon publish the numbers — pot life for a confined 100 g mass, working time for a thin film — and the shop rule is simple: small batches, right hardener for the temperature, pour out of the cup when you need time.
Materials
- WEST SYSTEM 105 Epoxy Resin (or equivalent marine epoxy) with the hardener matched to shop temp and job size — 205 Fast or 206 Slow for most boatshop work; 209 Extra Slow (or G/flex) when it's hot or the pour is deep
- Calibrated mini pumps or careful 5:1 by-volume metering for 205/206 (do not change the ratio to "slow it down")
- Clean plastic / metal / wax-free paper mixing pots — no foam, no glass
- Wide, shallow roller pan for coating jobs
- Filler (as needed) mixed in after resin and hardener are thoroughly blended
Tools
- Mixing sticks (scrape sides, bottom, and corners)
- Gloves, respirator / good ventilation when a pot starts to heat
- Shop thermometer — know whether you're closer to 60°F or 85°F before you pick a hardener
- Clamps / fillet sticks / scarf setup staged before you mix
Steps
- Pick hardener for shop temp and job, not for hope. At ~72°F, WEST SYSTEM 105/205 Fast: pot life (100 g) 9–12 minutes, thin-film working time 60–70 minutes, solid in thin film 6–8 hours (min temp 40°F). 105/206 Slow: pot life 20–25 minutes, working 90–110 minutes, solid 10–15 hours (min temp 60°F). Hotter shop or big laminate → reach for Slow / Extra Slow. Colder shop → Fast is often the right tool, and you'll mix a little longer.
- Meter the ratio — never "adjust" it for time. 205 and 206 are 5 parts resin : 1 part hardener by volume. Changing the hardener amount does not control cure speed; it creates a mix that may never fully cure and will have weak physical properties (Gougeon / Epoxyworks). Use pumps or marked straight-sided pots.
- Match batch size to the job. Fillets and scarfs: a few ounces, mix as you go. Spot bonding: same. Coating / glass: mix, then immediately pour into a roller pan so heat dissipates and thin-film working time applies. Large laminations: pre-measure several small batches ahead; mix one at a time; prefer 206 or 209. Do not cast thicker than ½″ in one pour (thinner if foam-insulated).
- Respect the 18°F rule and the cup. Every 18°F above 72°F roughly halves cure time (Epoxyworks). A confined mass builds its own heat — WEST notes a 100 g mass can reach ~400°F, and 8+ fl oz in a plastic cup can melt the cup and burn skin. Same quantity spread thin cures at ambient pace.
- Watch for kick warning signs — then stop using that pot. WEST calls the start of gel the "kick-off" / green stage: the mix thickens and flows slower, the cup may warm (and in a runaway can froth/smoke), then it goes tacky gel → hard rubber and is no longer workable. If the pot heats up: move it outdoors, don't breathe the fumes, wait until cool before disposal, and mix a fresh small batch. Don't try to "save" kicked epoxy on a scarf.
- Stage the work before you pump. Clamps open, fillet sticks ready, scarf faces wet-out planned. Pot life is a lab number for 100 g in a cup — your real clock is shop temp + how much is sitting in that pot. Work the clock; don't argue with it.
MOAM's shop note: Biggest miss is mixing one giant cup "so I don't have to stop" — then watching it kick while the second scarf is still dry. Second miss: running Fast hardener in an 85°F shop because that's what was on the shelf. Third: thinning the mix with extra hardener "to buy time" (it doesn't — it ruins the cure). Prefer small batches. Prefer the slow hardener when it's warm. Prefer pouring coating mixes into a pan. That's how epoxy stays a tool instead of a science experiment gone hot.
MOAM Out.










