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- Epoxy Resins
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When looking for a resin for your project, the variety can be overwhelming: epoxy, polyurethane, polyester, acrylic... Each type has distinct properties and responds better to some applications than others. In this guide we explain the key differences so you can choose wisely, without wasting time or material.
What Is a Resin and How Does It Work
Synthetic resins are liquid polymers that harden through a chemical reaction when you mix their two components: part A (base or resin) and part B (catalyst or hardener). This reaction is called polymerization, and its speed depends on the formulation, temperature and mixing ratio.
Once cured, they form a solid with properties far superior to most conventional materials: lightness, strength, durability and the ability to replicate any detail of the mold.
Epoxy Resin
Epoxy resin is the most versatile type with the best mechanical properties. It's characterized by its excellent adhesion to almost any surface, chemical resistance, low shrinkage during curing and stability against heat and UV rays.
What It's Used For
- High thickness castings: river tables, encapsulations, artwork. For thicknesses up to 12 cm in a single pour, ClearWater Deep Pour is the reference option at Feroca.
- Jewelry and transparent effects: ClearWater Epoxy for Jewelry offers exceptional transparency and resistance to yellowing.
- Structural laminates with fiberglass: Epofer Laminate 401 is formulated to impregnate fabrics and meshes, with 90 minutes of working time.
- Doming and raised finishes: the rigid and flexible versions of ClearWater Epoxy Doming allow you to create the characteristic resin drop on labels and accessories.
- Repair and adhesion: Epoxy Putty 610 adheres and fills on metal, wood, concrete and foam; polymerizes even underwater.
Advantages
- Low shrinkage when curing
- Excellent UV resistance in stabilized formulations
- Can be pigmented, filled and combined with fiberglass
- Food safe in specific formulations
Things to Consider
- More expensive than polyurethane or polyester
- Temperature sensitive: always work above 18-20 °C
- Some components can cause skin sensitization: always use nitrile gloves
Polyurethane Resin
Polyurethane resin is the preferred option when you need short curing times, low viscosity and a lightweight plastic finish. It comes in two liquid parts that, when mixed, react quickly to form a beige or white-looking solid.
What It's Used For
- Casting in silicone molds: the EasyFlo series —EasyFlo 60, EasyFlo 90 and EasyFlo 120— covers working times from 2.5 to 15 minutes and hardness from 65 to 70D. Ideal for replicating props, models and industrial parts.
- Rotomolding: EasyFlo 120 has a specific gradual cure for rotomolding, producing parts with high impact resistance.
- Two-part rigid molds: Sika Biresin F38 is perfect for counter-molds to pour silicone onto, with low shrinkage and no odor.
- Lightweight parts with buoyancy: Lik-Wood produces ultra-lightweight parts, designed for fishing lures and prototypes where weight matters.
- Pigmented castings: EasyFlo Clear is transparent before pigmenting, allowing sharper tones with translucent pigments.
Advantages
- Very fast curing times (from 15 minutes)
- Low viscosity: fills mold well and generates few bubbles
- Simple 1:1 mixing in many models
- Economical for production series
Things to Consider
- Very moisture sensitive: keep tightly closed and in a dry place
- Natural color (beige) can affect pigment tone in light colors
- Not suitable for large thickness castings in all models
Polyester Resin
Polyester resin is the most economical and widespread in fiberglass laminating work. It requires a catalyst in small amounts (typically 1 to 2% by weight) and cures at room temperature.
What It's Used For
- Laminates in marine, automotive and repairs: Ferpol 100 BSX15 is a classic laminating resin with good mechanical properties for reinforcing parts with fiberglass.
- Decorative castings: Ferpol 3501 CV2.5 is formulated for casting, with good finishing properties.
- Inclusions and translucent effects: Ferpol 1973 Transparent Resin is used to encapsulate objects with a translucent result.
- Crack repair in pools, tanks and hulls: thanks to its adhesion on GRP and water resistance.
Advantages
- Lower price than epoxy
- Extensive track record in naval and automotive industry
- Configurable working times by adjusting catalyst percentage
- Compatible with polyester gel coat for surface finishes
Things to Consider
- Higher shrinkage when curing than epoxy
- Strong odor: always work in well-ventilated spaces
- Lower chemical and UV resistance without additional treatment
- Not suitable for use on foam: styrene dissolves expanded polystyrene
Acrylic Resin
Acrylic resin is the solvent-free and VOC-free, water-based alternative, ideal when the work environment doesn't allow the use of resins with odor. It's especially popular in decoration, crafts and manufacturing ornamental pieces.
What It's Used For
- Decorative castings and molded parts: Jesmonite AC100 mixes at 2.5:1 ratio (powder:liquid) and cures in about 10 minutes, producing plaster-like but much stronger pieces.
- Cement and stone effect: Jesmonite AC730 allows imitating concrete, stone or metal finishes, widely used in interior decoration and props.
- Foam coating: acrylic resin doesn't attack foam, unlike polyester, so it's the only safe option for coating foam figures and models.
- Laminates with fiberglass indoors: can be reinforced with glass fabric to create panels and housings without VOCs.
- Projects in workshops and educational environments: no odor or solvents, safe with normal ventilation.
Advantages
- No solvents or VOCs: no odor, no special ventilation restrictions
- Compatible with foam without dissolving the substrate
- Fire and impact resistant (AC100 and AC730)
- Easy cleanup with water before curing
Things to Consider
- Lower mechanical strength than epoxy in structural uses
- Not suitable for prolonged water contact outdoors without protective varnish
- Natural opaque white color: not transparent
Which to Choose? Comparison Table
| Characteristic | Epoxy | Polyurethane | Polyester | Acrylic |
|---|---|---|---|---|
| Transparency | High (clear formulations) | Medium (beige/white) | Medium-low | None (opaque white) |
| Mechanical strength | Very high | High | Medium | Medium |
| UV resistance | High (stabilized) | Medium | Low | Medium |
| Curing speed | Medium (hours) | High (minutes) | Medium | Fast (10 min) |
| Relative price | High | Medium | Low | Medium |
| Odor / VOC | Low | Low-medium | High (styrene) | No odor / no VOC |
| Suitable for foam | Yes | Yes | No (dissolves) | Yes |
| Large thickness castings | Yes (Deep Pour) | No | No | No |
Frequently Asked Questions
What's the difference between epoxy and polyurethane resin for making replicas in silicone molds?
Polyurethane cures in minutes and is more economical for series, but its natural color (beige or white) limits transparency possibilities. Epoxy takes longer to cure and is more expensive, but offers greater mechanical strength and better transparency results when using clear formulations. For opaque colored replicas with repetitive production, polyurethane is unbeatable in speed; for high-quality pieces or with complex visual effects, epoxy is the best choice.
Can I use polyester resin on foam?
No. Polyester contains styrene, which dissolves foam (expanded polystyrene) on contact. To coat foam, use acrylic resin like Jesmonite AC100 or epoxy resin, which don't attack the substrate. If you want to laminate with fiberglass over a foam model, first impregnate the foam with a layer of epoxy or acrylic and, once cured, laminate on top.
What's the most suitable resin for a river table?
For a river table you need transparency, UV resistance and the ability to make large thickness castings without the resin overheating. ClearWater Deep Pour is designed exactly for this: up to 12 cm casting in a single session, low exotherm and yellowing resistance. Polyurethane, polyester and acrylic are not suitable for this use.
What resin is best for working indoors without special ventilation?
Acrylic resin —Jesmonite AC100 or AC730— is the odor-free and VOC-free option par excellence. Epoxy resins also have relatively low odor compared to polyester, but ventilation during mixing and curing is always recommended. Polyester emits styrene and requires forced ventilation. If you work in a small space without extraction, acrylic is the safest choice.
Can I mix resin types in the same project?
Yes, in some cases it's a common technique. For example, a first layer of epoxy on the mold to capture surface detail, and a rigid polyurethane backing to provide structure and reduce costs. The key is that each layer is well cured before applying the next. Never mix the liquid parts of different systems: always respect the A+B pair of each resin.
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