Reach for the wrong resin and even a perfect pour can fail — yellowing, cracking, or staying soft for days. The single most common beginner mistake isn't technique; it's pouring the wrong type of resin for the job. Casting resin comes in three flavours, and each is built for a different depth and speed. This guide explains what casting resin actually is, compares the three types side by side, and tells you exactly which to grab for your project.
What is casting resin, and how is it different from coating resin?
Casting resin is a thin, slow-curing epoxy made for pouring into molds, while coating (or doming) resin is thicker and made for self-levelling over flat surfaces. That difference in viscosity is everything: casting resin flows into detail and can cure in depth, while coating resin sits in a shallow layer and overheats if you pour it thick.
Put simply — if you're filling a mold, you want casting resin. If you're glazing the top of a tabletop or a painting, that's coating resin. Using the wrong one is behind a surprising number of "why did my resin fail?" posts. Casting resin is what you'll pour into shapes from our resin molds collection.
The three types of casting resin, compared
There are three casting resins — slow-cure, fast-set, and deep-pour — and they differ mainly in pot life and how deep you can pour them in one go. Here's the side-by-side the spec sheets rarely give you. Treat the numbers as typical ranges; your product's label is always the final word.
| Type | Pot life | Demold time | Max single pour | Hardness | Best for |
|---|---|---|---|---|---|
| Slow-cure casting | 30–40 min | 24–48 hrs | ~10–15 mm layers | Hard (85–90 Shore D) | Shallow decorative casts, clear pieces |
| Fast-set casting | 15–20 min | 12–24 hrs | Thin, small pieces | Firm | Jewellery, small detail, quick turnaround |
| Deep-pour | 4+ hrs | 36–48 hrs | Up to ~50 mm (2 in) | Slightly softer (78–82 Shore D) | Deep molds, spheres, river tables |
Full cure — the point where resin reaches maximum hardness — takes about 7 days for slow-cure and deep-pour resins, even though you can demold much sooner. Plan your finishing around that, not around demold time.
When should you use slow-cure casting resin?
Use slow-cure casting resin for shallow, detailed decorative pieces where you want the hardest, clearest finish. Its longer 30–40 minute pot life gives you time to colour, layer, and chase out bubbles, and it cures to a tough 85–90 Shore D that sands and polishes beautifully. The trade-off is patience — a full week to reach maximum hardness.
This is the everyday workhorse for coasters, trinkets, trays, and jewellery dishes. The long working time is forgiving for beginners still finding their rhythm — the same qualities that make our guide to making resin coasters a good first project.
When should you use fast-set casting resin?
Use fast-set resin when you want small pieces demoulded and finished quickly. With a short 15–20 minute pot life and a 12–24 hour demould, it's built for momentum — jewellery, pendants, and small detailed casts you want to turn around in a day. It also tends to yellow less on tiny, thin pieces because there's little mass to overheat.
The catch is speed: 15 minutes goes fast, so mix small batches and have your colours and mold ready before you start. It's the natural match for high-volume small work like keychain molds, where a quick cure means more pieces per session.
When should you use deep-pour resin?
Use deep-pour resin any time your cast is thicker than about 15 mm in a single pour. Standard casting resin poured too deep overheats — epoxy gives off heat as it cures, and a thick mass of fast resin can hit temperatures that make it yellow, crack, or even smoke. Deep-pour is formulated to cure slowly, releasing that heat gently, so you can pour up to around 50 mm in one go.
I learned this the hard way with a resin sphere: I filled it 3 cm deep with ordinary art resin, and by the next morning it had ambered and cracked from the inside out. One switch to deep-pour resin and the next sphere came out flawless and glass-clear. For thick geometric casts, pair deep-pour resin with a mold built for depth like our deep-pour geometric mold.
Does temperature affect how casting resin cures?
Yes — casting resin cures best around 21–24°C (70–75°F), and both cold and heat cause problems. Below roughly 18°C the reaction slows and can leave a tacky or cloudy surface; too warm, or too thick a pour, and the exotherm runs away into yellowing and cracks. Warm your resin bottles in warm water for a few minutes before mixing so it flows thin, and cure in a stable, dry, room-temperature space.
Why did my resin yellow, stay soft, or crack?
Most casting failures trace back to the wrong resin type, an off ratio, or the wrong temperature. Match the symptom to the cause below before you blame the resin itself.
| Problem | Likely cause | The fix |
|---|---|---|
| Yellowed or ambered | Standard resin poured too thick, or UV exposure | Use deep-pour for depth; keep pieces out of sun |
| Cracked from the inside | Overheated thick pour (exotherm) | Deep-pour resin, or build up in thin layers |
| Stayed soft or bendy | Off mix ratio, or a cold room | Measure exactly; cure at 21–24°C |
| Cloudy or milky | Moisture or too-cold cure | Dry the mold; warm the room and resin |
| Trapped bubbles | Poured cold or too fast | Warm the resin; pour thin and slow |
If your issue is the ratio, our guide on resin mix ratios has the numbers; if it's surface bubbles or pinholes, see how to prevent resin surface bubbles.
What mold and safety basics do you need?
Cast resin in a flexible silicone mold, work in a ventilated space, and wear gloves. Platinum silicone releases cured resin cleanly with a flex, no release agent required, and captures fine detail that rigid molds round off. On safety: uncured epoxy can irritate skin and shouldn't be inhaled, so ventilate, wear nitrile gloves, and avoid contact — but once fully cured, epoxy is inert and safe to handle.
- Mold: flexible platinum silicone for clean release and sharp detail.
- Ventilation: an open window or extraction while mixing and pouring.
- PPE: nitrile gloves; avoid skin contact with liquid resin and hardener.
- Storage: keep resin sealed, away from cold and sunlight; most stays usable about 12 months.
Frequently Asked Questions
What is the difference between casting resin and coating resin?
Casting resin is thin and made for pouring into molds, curing in depth; coating (or doming) resin is thicker and made to self-level in a shallow layer over flat surfaces. Use casting resin to fill a mold and coating resin to glaze a tabletop or artwork. Pouring coating resin thick makes it overheat and fail.
How deep can you pour casting resin?
It depends on the type. Standard casting resin should be poured no deeper than about 10 to 15 mm per layer, or it overheats and yellows. Deep-pour resin is formulated to release heat slowly and can go up to around 50 mm (2 inches) in a single pour. For anything thick, always choose a deep-pour formula.
Which casting resin cures the fastest?
Fast-set casting resin cures fastest, with a 15 to 20 minute pot life and a 12 to 24 hour demould. It suits small, detailed pieces like jewellery where you want quick turnaround. Slow-cure and deep-pour resins take 24 to 48 hours to demould and about 7 days to reach full hardness, trading speed for depth and clarity.
Why did my resin turn yellow?
Yellowing usually means a standard resin was poured too thick and overheated, or the finished piece sat in sunlight. Epoxy releases heat as it cures, and a thick mass gets hot enough to amber. Use a deep-pour resin for depth, pour in thin layers if needed, and keep cured pieces out of direct UV light.
What temperature should you cure resin at?
Cure casting resin at around 21 to 24°C (70 to 75°F) in a dry space. Below roughly 18°C the resin cures slowly and can stay tacky or turn cloudy, while excessive heat or too-thick a pour causes yellowing and cracks. Warm the bottles before mixing so the resin flows thin and releases bubbles more easily.
Got your resin sorted? Pair it with the right shape — browse flexible, high-detail designs in our resin mold collection and pour with confidence.
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