Why the surface coat matters
A moulded FRP part is built from the outside in: gelcoat first, then laminate. The gelcoat becomes the outer surface, and in a fire it is the layer exposed first. On rail interiors, cab mouldings, bus and coach panels and similar parts, a fire-retardant laminate behind an ordinary gelcoat is an incomplete system.
The 2026 catalogue lists two fire-retardant gelcoats: SPR 201 FR-GC in the orthophthalic family and SPR 401 FRGC in the terephthalic family.
Questions to settle
Before the first trial
- Which fire standard and classification does the finished part need?
- Which fire-retardant laminating resin will be used behind the gelcoat?
- Brush or spray application, and what equipment?
- What colour, and must it match an existing fleet or interior?
- Is there a separate requirement for surface appearance, cleaning or graffiti removal?
Choosing between the two gelcoats
The two differ in chemistry and in how they are specified. SPR 201 FR-GC is catalogued with high- and low-shear viscosities and a reference cure of MEKP at 1.5–2.0%. SPR 401 FRGC is 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%.
In practice the choice follows the laminate system the part uses, the application equipment and the result of testing the complete build. The ortho versus tere FR gelcoat comparison sets this out as a decision table.
Published typical values
Viscosities for the two gelcoats are measured under different conditions and should not be compared number for number.
| 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 |
| 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.
Application points to trial
Gelcoat quality is decided by film thickness, evenness, and timing. Too thin a film exposes the laminate pattern and reduces the protective layer; too thick a film risks cracking. Vertical faces and returns are where sag shows. The laminate must be applied when the gelcoat has reached the right state — tacky but not transferring — and that timing depends on temperature.
Establish film thickness, gel and back-up timing on the production mould and equipment, and keep them consistent in production, because the tested build assumes them.
Testing and documentation
Neither fire-retardant gelcoat carries a published fire classification. The classification comes from testing the finished construction — gelcoat, colour, laminate, thickness and any core together. Keep a record of the materials and batch numbers used for the test panels so production can be shown to match them.