Epoxy cures by a chemical reaction between resin and hardener. If it’s still tacky or soft well past the stated cure time, something went wrong with that reaction. The good news is that the cause is usually easy to pin down.
Diagnose the pattern
- Sticky everywhere: likely the wrong ratio, the wrong hardener, or too cold.
- Soft or sticky patches in an otherwise hard surface: classic poor mixing. Unmixed resin or hardener scraped from the side of the cup ended up on the work.
- Hard but with a greasy or waxy film: probably amine blush, not under-cure. See amine blush on epoxy.
Common causes
- Off-ratio mixing. Epoxy systems are formulated for a specific ratio. Adding extra hardener does not make it cure faster or harder. Measure by volume or weight exactly as the product says.
- Not enough mixing. Mix for the full recommended time and scrape the sides and bottom. Some people pour into a second cup and mix again.
- Low temperature. Cure slows a lot in the cold. Below a product’s minimum temperature it may stop and never fully cure. See curing epoxy in cold weather.
- Contamination or moisture on the surface or in the tools.
- Old or crystallized resin that wasn’t warmed back to clear before use (see crystallized resin).
How to fix it
- If it’s just slow from the cold, warm the area and give it more time.
- If it’s truly uncured, scrape off as much soft material as you can, clean the residue with a solvent the manufacturer recommends, sand, and recoat with properly mixed epoxy.
- Don’t pour a new coat over a soft one. It won’t fix the layer underneath.
Product-specific cure schedules and cleanup advice are in the manufacturer’s literature, such as the WEST SYSTEM instruction manuals, and in technical articles on Epoxyworks. Related: epoxy safety basics