Quick facts
- Chemistries compared
- Orthophthalic · isophthalic · bisphenol-A epoxy vinyl ester
- Representative grades
- GP Clear (SPR 201 GPC) · ISO Polyester (SPR 301) · Vinyl Ester (SPR 501 VE)
- Specialised isophthalic lining grades
- SPR 301 OT (oil tanks) · SPR 301 OS (organic solvents, effluent)
- Deciding factor
- What the lining will be in contact with, and for how long
Three generations of lining chemistry
Unsaturated polyesters are built on a dibasic acid, and the acid changes how the cured laminate copes with water and chemicals. Orthophthalic grades such as GP Clear Resin are economical and widely used for general FRP. Isophthalic grades such as ISO Polyester Resin build a backbone that resists the slow attack of water on ester links better, which is why the industry uses them for tanks and pipework in wet or chemically loaded service. Vinyl ester is a separate chemistry, generally positioned above isophthalic polyester for aggressive chemical service.
This guide is about that choice of chemistry. The companion guide on SPR 301 OT, SPR 301 OS and Vinyl Ester compares the specialised lining grades once the chemistry is settled.
How to decide
If the laminate will not hold water or chemicals in long-term contact — a cover, a dry housing, a structural shell — an orthophthalic general-purpose grade may be the sensible specification. If the lining will hold water or chemicals within the isophthalic range for years, move to an isophthalic grade, and to SPR 301 OT or SPR 301 OS where the catalogue lists that exact service. If the chemical is aggressive, the concentration or temperature is high, or the owner’s specification names vinyl ester, use Vinyl Ester Resin.
A tank can also be built in layers: the chemistry that faces the contents decides the lining, and the structural wall behind it is specified separately by the tank designer.
What the data does not tell you
The published values describe the liquid resin — viscosity, gel time, styrene — not how a finished lining performs against a particular chemical. None of the three records publishes a compatibility chart. The ISO Polyester heat deflection temperature (90 °C) and the Vinyl Ester thermal figure (110 °C) are resin data, not service-temperature limits for a lining.
GP Clear Resin’s viscosity and gel time are published without a stated test temperature, while the other two state 25 °C, so the rows are indicative rather than a like-for-like measurement.
Sources, roles and the Nepal enquiry path
GP Clear Resin, ISO Polyester Resin and Vinyl Ester Resin values come from each product’s supplier documentation as published on this site. SPR 301 OT and SPR 301 OS are referred to from the supplier’s 2026 product catalogue.
Samrat Poly Resins, India manufactures the grade and issues its technical information. Samrat FRP Traders is the Nepal supplier and importer and handles enquiries; price, packaging, lead time and stock in Nepal are confirmed on enquiry.
Three chemistries for a tank lining
| Criterion | GP Clear (ortho) | ISO Polyester | Vinyl Ester |
|---|---|---|---|
| Chemistry | Orthophthalic unsaturated polyester | Isophthalic unsaturated polyester | Bisphenol-A epoxy vinyl ester |
| Documented role | General-purpose FRP: hand lay-up, spray-up, water tanks, roofing sheets, panels | FRP work where chemical resistance, water resistance and long-term durability matter | Industrial FRP where chemical and corrosion resistance is part of the selection |
| Viscosity | 350–450 cPs | 500 cPs at 25 °C | 350 cPs at 25 °C |
| Gel time | Under 8 minutes | 15 minutes at 25 °C | 15–20 minutes at 25 °C |
| Styrene content | 33–40% | 40% by weight | 40% |
| Supply | Non-accelerated; MEKP and cobalt octoate dosed at the workshop | Non-accelerated; MEKP catalyst with cobalt octoate accelerator | Cure system as stated in the supplier TDS |
| Place in a tank lining | Dry or lightly loaded service; not the chemistry chosen for chemical contact | Water and moderate chemical service; SPR 301 OT / OS where the catalogue lists the exact service | Aggressive chemicals, higher concentration or temperature, or where vinyl ester is specified |
| Choose when… | The laminate will not hold water or chemicals in long-term contact | The lining faces water or chemicals within the isophthalic range | The service is beyond what an isophthalic lining is specified for |
Sources: each product’s supplier Technical Data Sheet and site data (GP Clear Resin, ISO Polyester Resin, Vinyl Ester Resin); SPR 301 OT and OS are described in the Samrat Poly Resins, India product catalogue, 2026 edition. Typical values describe each grade; they are not batch guarantees or acceptance limits.
Frequently asked questions
Can GP Clear Resin be used to line a chemical tank?
It is a general-purpose orthophthalic grade. Where a lining holds water or chemicals for years, the industry moves to an isophthalic or vinyl ester chemistry; the service conditions decide which.
Does “ISO” in ISO Polyester Resin mean a certification?
No. It means isophthalic acid, the dibasic acid the resin is built on. It is not an ISO certification or standard.
Where do SPR 301 OT and SPR 301 OS fit?
They are isophthalic grades listed for specific services: OT for oil-storage tanks and chemical equipment, OS for organic-solvent storage, sewage pipes and effluent equipment.
Is vinyl ester always the safer choice?
Not automatically. It is the route for service beyond what an isophthalic lining is specified for. For milder service an isophthalic grade may meet the owner’s specification; the actual chemical, concentration and temperature decide.