How to Post Cure an Epoxy Composite Part: Temperature and Schedule
30 October 2026
To post cure an epoxy composite part you heat it, after it has hardened at room temperature, to a controlled temperature above its cure temperature and hold it for hours. That finishes the crosslinking a room-temperature cure leaves undone and raises the glass transition temperature (Tg), the number that decides whether a dark deck goes soft in July sun. For West System 105 with 205 or 206 hardener, our schedule is 24 hours at 70°F (21°C) until the part is hard, a slow ramp to 120 to 140°F (49 to 60°C), a 4 to 8 hour hold, and a slow cool. Here is when it pays and how to run it without warping the part.
What a post cure actually changes
Crosslinking slows to a crawl once the resin's Tg climbs up to the temperature it is sitting at, so a laminate cured at 70°F stalls with a Tg not far above that. Heat it and the reaction restarts, and Tg climbs to the ceiling the chemistry allows, typically 120 to 140°F for a room-temperature laminating epoxy; the exact figure is on the TDS. A post cure also gets you to full strength in a day instead of the 1 to 4 days West System quotes for 205 and 206 at 72°F, and it takes out the print-through that shows up the first time a part heats in the sun.
When it pays and when to skip it
Post cure when the part will see heat: dark hulls and decks, engine covers, anything painted black and baked in a booth, and tooling molds that will have exothermic parts pulled from them. Post cure when you need to sand or fair the part tomorrow, or when a 206 layup has been sitting leathery in a cold shop for two days.
Skip it for interior parts that never see 100°F, for bonded joints and fillets that finish curing over weeks of ambient heat, and for anything you cannot support at temperature. PVC-cored parts stay at 140°F (60°C) or below, because the foam softens as it approaches 160°F (71°C) and the skins settle into it. Wood-cored and plywood parts stay at 120°F (49°C) or below, or the wood outgasses under the skin and prints through.
Epoxy post cure temperature and schedule
| Stage | Temperature | Time | Notes |
|---|---|---|---|
| Initial cure | 70°F (21°C) or above | 24 h with 205; 24 to 36 h with 206 | Part hard, still on the mold or a fixture |
| Ramp up | 10 to 15°F (5 to 8°C) per hour | 4 to 5 h to reach 140°F | Faster and the part outruns its Tg and creeps |
| Hold | 120 to 140°F (49 to 60°C) | 4 to 8 h | Measure at the part, not the air; the TDS sets the ceiling |
| Cool down | 15°F (8°C) per hour or slower | 3 to 4 h to under 100°F (38°C) | Demold when cool to the hand |
The ramp is slow because a part passing its current Tg goes leathery, and unsupported at that moment it sags and keeps that shape. Leave it on the mold or a matching fixture, and if it was bagged, leave the bag on with the pump running through the whole cycle. Set the hold 15 to 25°F above the hottest service temperature, capped by the TDS maximum; a black hull in Florida sun reads 150°F (65°C) at noon. For a bagged deck hatch in 105/206 we ramp 10°F an hour to 130°F, hold 6 hours, and cool overnight in the box with the heater off.
Hardener choice with a post cure in mind
Both West System hardeners mix 5:1 by volume with 105 resin; pick between them on pot life and shop temperature. West System 205 Fast Hardener gives 9 to 12 minutes of pot life in a 100 g cup at 72°F (22°C), reaches a solid state in 6 to 8 hours and keeps curing down to 40°F (4°C): the cool-shop and small-batch choice. West System 206 Slow Hardener gives 20 to 25 minutes in the same cup, is solid in 10 to 15 hours and wants at least 60°F (16°C): the pick for big layups, warm days and glue-ups. Pot life shortens with mass, so mix in wide, shallow White Paper Mixing Tubs and get the batch onto the laminate fast; a full 32 oz tub of 205 left standing will smoke.
Heat sources and how to measure
A hot box of foil-faced foam board with a small fan-forced space heater and a plug-in thermostat holds 140°F; for a hull, a poly tent over a frame does the same. Heat lamps make hot spots, 180°F under the lamp and 90°F at the far end, so keep them 3 ft (1 m) off the part and moving. Run the schedule off a probe thermometer against the laminate under a scrap of breather; box air leads part temperature on the ramp. For bagged parts keep the vacuum on: Spiral Vacuum Tubing - 1/2" OD x 3/8" ID under the breather spreads vacuum along the part, and a run of Vacuum Tubing - 1/2" OD x 3/8" ID - 100ft Roll gets the line out of the box to the pump.
The mold has to survive the same cycle: tooling never taken above 120°F will move at 140°F with a part locked to it, so run new molds through their own post cure empty first. Build the release with Partall #2 Paste Wax, several thin coats buffed between, and check its data sheet for a temperature limit first. Wear a fitted P100 respirator with organic vapor cartridges when mixing and sanding epoxy, nitrile gloves and eye protection.
Frequently asked questions
What temperature should I post cure epoxy at?
For West System 105 with 205 or 206 and similar laminating epoxies, hold 120 to 140°F (49 to 60°C) for 4 to 8 hours after a 24 hour cure at 70°F, ramping no faster than 15°F per hour. Sit 15 to 25°F above the hottest temperature the part will see, never above the TDS maximum, and keep cored parts at 140°F or below.
How soon after layup can I post cure a composite part?
Not until it is hard enough to hold its own shape: 24 hours at 70°F with 205, 24 to 36 hours with 206. Heat a green laminate and it sags into whatever shape it is lying in. Keep it on the mold for the whole cycle, and if it was bagged, keep the pump running until it is back under 100°F.
Does West System 105/205 need a post cure?
Not for most boat work. 105/205 is solid in 6 to 8 hours at 72°F and at full strength in 1 to 4 days on its own, and a white hull or an interior bond never gets hot enough to matter. Post cure it when the part is dark, sits near an engine, or gets a paint booth bake.
Can I post cure a part in the sun or in a closed car?
A black plastic tent in Florida sun will hit 130°F, but you cannot control the ramp, and a part that jumps from 75°F to 130°F in half an hour before it is fully hard will warp. If sun is your only heat, wait 48 hours after layup, keep the part on its mold, and put a probe thermometer on it.
Will post curing warp my part?
Only if it is unsupported or heated too fast. Every part goes leathery as it passes its current Tg, so a hatch lid lying loose on a bench at 130°F takes a set. Leave it on the mold, ramp 10 to 15°F per hour, cool at the same rate, and post cure the mold itself first, or the mold moves and the part with it.
What You Need For This Job
- West System 205 Fast Hardener: 5:1 fast hardener for cool shops and small batches
- West System 206 Slow Hardener: 5:1 slow hardener for big layups and warm shops
- White Paper Mixing Tubs: wide, shallow tubs that keep a batch from cooking off
- Partall #2 Paste Wax: release for tooling that rides through the hot cycle
- Spiral Vacuum Tubing - 1/2" OD x 3/8" ID: spreads vacuum along the part under the breather
- Vacuum Tubing - 1/2" OD x 3/8" ID - 100ft Roll: the line from the bag out of the box to the pump
