Two fire-retardant surfaces
Both gelcoats are listed for rail and transport and automotive composite components, both are thixotropic and supplied non-accelerated, and neither carries a fire classification of its own. They belong to different resin families and are specified differently in the catalogue.
Where they differ
SPR 201 FR-GC is orthophthalic, milky white to grey, catalogued with high- and low-shear viscosities and a reference cure of MEKP at 1.5–2.0%. SPR 401 FRGC is terephthalic, supplied in white, grey or custom colour, catalogued with viscosities at 2 and 20 rpm, a thixotropic index of 4.5–6.0 and a reference cure of MEKP 1.5% with cobalt octoate 0.4%.
Published typical values
The two gelcoats’ viscosities are measured under different conditions, so the figures should not be compared directly.
| Property | Orthophthalic Fire-Retardant Gelcoat Resin (SPR 201 FR-GC) | Terephthalic Fire-Retardant Gelcoat Resin (SPR 401 FRGC) |
|---|---|---|
| Chemistry | Orthophthalic unsaturated polyester gelcoat, fire-retardant | Terephthalic unsaturated polyester gelcoat, fire-retardant |
| Appearance | Milky white to grey opaque liquid; custom-tinted variants where supplied | Opaque, thick pigmented liquid |
| Viscosity (typical) | 5,000–11,000 cPs at 25 °C (High Shear); 30,000–45,000 cPs at 25 °C (Low Shear) | 12,000–18,000 cPs (2 rpm); 2,000–3,500 cPs (20 rpm) |
| Thixotropy | Not published | 4.5–6.0 |
| Gel time / pot life (typical) | 9–16 minutes at 25 °C | 12–22 minutes at 25 °C |
| Specific gravity / density (typical) | 1.35–1.55 | 1.25–1.45 g/cm³ |
| Styrene content (typical) | 17–25% by weight | 28–35% by weight |
| Reference cure | MEKP 1.5–2.0% — added during use | MEKP 1.5% + Cobalt Octoate 0.4% — added separately during use |
| Source of values | 2026 catalogue; grade TDS | 2026 catalogue; grade TDS |
Typical values, reproduced from each product’s own published data. They describe the liquid resin at its reference condition, not a finished part, and are not guaranteed limits.
How to decide
Match the gelcoat to the laminate system and the application equipment, agree the colour, then test the complete construction to the project’s fire standard. If a construction has already been tested, the gelcoat in it is the one to keep.