Resin

Why your resin is still sticky: ratio, temperature and mixing

A piece still tacky a day later is the most common resin failure, and it has few causes — nearly all decided in the first five minutes: how you measured, how you mixed, and how warm the room was.

Still curing, or never will?

Still curing is firm but slightly tacky, or hard in the middle and soft at the edges, and it improves each time you check. Most casting and coating epoxies are touch-dry in 12-24 hours, handleable at 24-48 and workably hard by about 72; full chemical cure can take a week, and a thin coat cures more slowly than a deep pour because it generates less heat. Give it the full 72 hours somewhere warm before concluding anything.

Will never cure is resin that is still liquid, permanently rubbery, greasy, or dented by a fingernail after several days at a proper temperature. A bad ratio does not fix itself with time or with heat: the chemistry needs a specific proportion of resin to hardener, and whichever part you over-poured has nothing left to react with. Scrape it off and pour again — a fresh coat over uncured resin only traps the problem underneath.

Cause one: the ratio

The commonest mistake is measuring by volume when the product is specified by weight, or the reverse. A resin sold as 1:1 by volume is frequently not 1:1 by weight, because the parts differ in density; the by-weight figure might be something like 100:83. Follow whichever method the data sheet gives.

• Weight ratio: use a scale reading to 0.1 g and tare between parts. Volume ratio: use graduated cups, read at eye level.
• Never eyeball a partial batch; halving a mix is where errors creep in.
• Scrape both cups out. Hardener left clinging inside is a ratio error that happens after you measured correctly.
• Extra hardener does not speed the cure. It leaves unreacted material behind and gives a soft, tacky result.

Cause two: under-mixing

Epoxy has to be mixed thoroughly, not stirred until it looks uniform. What catches people out is the sides and bottom of the cup, where a film of unmixed resin or hardener clings and goes into the work at the end. The double-cup method solves it:

1. Mix the measured parts in the first cup for two to three minutes, scraping the sides and base with a flat stick.
2. Pour into a second clean cup, letting the first drain but not scraping the last of it in.
3. Mix again for one to two minutes with a clean stick.

Mix slowly: fast stirring beats air in. Adding time to a single-cup mix does not help, because the film on that cup's wall never gets mixed.

Cause three: working too cold

Epoxy cure is exothermic: the reaction generates heat, and that heat drives it on. Below roughly 21-24 °C (70-75 °F) it slows sharply, enough that a thin layer may never generate the warmth it needs and stalls part-cured. Cold resin is also thicker, so it levels slowly, mixes badly and traps bubbles.

Aim for 22-25 °C (72-77 °F) and hold it for the whole cure, not just the pour; warm the mould too, because pouring warm resin into cold silicone pulls the heat back out.

Cause four: humidity and amine blush

The amine hardeners in many systems react with moisture and carbon dioxide in the air to form a waxy, greasy film: amine blush. It looks tacky even though the resin underneath has cured perfectly well.

It is a surface problem with a surface fix: wash with warm water and washing-up liquid, dry thoroughly, then sand and recoat. Do not sand first — that drives the film into the scratches and the next coat will not bond. Work below about 70-75% relative humidity where you can.

Bubbles

Bubbles come from three places: air stirred in while mixing, air out of a porous embedded item, and cold thick resin that cannot release what it trapped.

• Warm the bottles before measuring so the resin is thinner and bubbles can rise.
• Mix with the stick below the surface, and pour slowly in a thin stream down a stir stick or the side of the mould.
• Seal porous inclusions — dried flowers, paper, wood — with a thin coat of resin and let it cure before embedding them.
• Let the pour stand a few minutes so bubbles rise on their own.

Then use gentle heat. A pass with a heat gun on low, or a small torch flame kept moving several centimetres above the surface, breaks the surface tension and pops what has risen. Keep it brief: blasting the surface overheats the resin and causes yellowing, dimples and fisheyes.

Fixing a tacky top layer

If the piece is sound underneath and only the top coat is soft, you can usually save it. Scrape and wipe off everything still soft or greasy with isopropyl alcohol or acetone on a rag, rather than sanding wet resin, which only smears it. If the problem was blush, wash with warm soapy water first.

Once down to a hard, dry surface, sand to a uniform matte with a medium grit: the point is a key for the new coat, not a polish. Wipe the dust off, then recoat with a fresh, correctly measured, properly mixed batch in a warm room. If the softness goes right through, no recoat will fix it.

Safety

Uncured epoxy is a sensitiser: repeated skin contact can make you allergic to it, and once sensitised you may not be able to work with it at all.

• Wear nitrile gloves, not latex, which protects poorly against epoxy; change them when they get resin on them.
• Work in a well-ventilated space, and keep resin off your skin.
• Wear a properly fitted respirator when sanding cured resin; the dust is fine and irritant.
• Never pour resin, hardener or rinsings down a drain: mix leftovers, let them cure to a solid block and bin it, or take unmixed leftovers to a hazardous waste facility.
• Keep resin away from children and pets, and store both parts closed above the label's minimum temperature.

The resin workshop box is in development, planned around a weight-ratio system with a scale, because the ratio is where most first attempts fail. Other planned boxes are on the boxes page.