The short answer
Start from what the finished part will face, not from the resin name. General-purpose (GP, orthophthalic) polyester covers dry or indoor parts and everyday hand lay-up. Isophthalic (ISO) polyester is specified where a laminate sits in water, chemical service or long outdoor exposure. Vinyl ester is for corrosion-resistant linings and chemical-service structures. Two-part epoxy, a resin mixed with its own hardener, is for casting, art and decorative surfaces, and fibreglass lamination.
The first three families cure the way polyester does, with an MEKP catalyst. Epoxy cures only with its own hardener. So the cure system your workshop already runs is part of the choice.
The four families side by side
| Family | What it is | Where the supplier specifies it | Cure | Products here |
|---|---|---|---|---|
| Orthophthalic (GP) polyester | General-purpose unsaturated polyester (UPR), built on phthalic anhydride. | Everyday FRP lay-up, casting, sheet and moulding compounds. | MEKP; non-accelerated grades also need an accelerator — confirm the cure system with us. | Unsaturated Polyester Resin / GP Clear Resin, Lamination Resin (LR), GP Yellow Resin — all polyester grades |
| Isophthalic (ISO) polyester | Polyester built on isophthalic acid instead of phthalic anhydride. “ISO” means isophthalic acid, not an ISO certification. | Laminates in water immersion, chemical service or sustained outdoor conditions. | MEKP; the ISO Polyester Resin TDS states non-accelerated. | ISO Polyester Resin and the special ISO grades — ISO Resins |
| Vinyl ester | An epoxy resin reacted with acrylic or methacrylic acid and dissolved in styrene. It has an epoxy backbone but is not an epoxy. | Corrosion-resistant linings and chemical-service structures; it sits between polyester and epoxy in chemical resistance, toughness and cost. | Like a polyester (MEKP). | Vinyl Ester Resin (Bisphenol-A epoxy vinyl ester) |
| Epoxy | A two-part system: resin mixed with a hardener. | Clear casting, art and decorative surfaces, and fibreglass hand lay-up and lamination — each by its own TDS. | With an epoxy hardener (here a polyamide type) — not with MEKP. | Clear Casting Resin, Epoxy Art Resin, Fiberglass Epoxy Resin, Epoxy Hardener |
Epoxy floor-coating systems are not part of this range. The epoxy products here are for casting, art and decorative surfaces, and fibreglass lamination.
Where the names overlap
- UPR and GP Clear: Samrat Poly Resins' documentation confirms that the product sold as “Unsaturated Polyester Resin” is GP Clear Resin.
- LR resin: short for Lamination Resin. Gel time on its TDS: 5 minutes.
- GP Yellow: a non-accelerated general-purpose grade. Its TDS gives a gel time of 10–15 minutes, slower than GP Clear (< 8 minutes). See GP Clear vs GP Yellow.
- ISO resin: isophthalic polyester, not a certificate — what ISO resin means.
- Polyester vs epoxy clear casting: Polyester Clear Casting Resin (SPR 201 CC) is a polyester; Clear Casting Resin is an epoxy. See the difference.
Three questions that decide the family
- What will the finished part face? Dry or indoor use, long water contact or outdoor exposure, or aggressive chemicals? The answer points to GP, ISO or vinyl ester. Compare ISO vs GP, ISO vs vinyl ester and UPR vs vinyl ester.
- Which process? Hand lay-up and spray-up, casting, or decorative coating? For lay-up, see GP vs lamination resin. For casting, see polyester vs epoxy casting.
- Which cure system does your workshop run? On an MEKP-cured polyester line, vinyl ester cures the same way. Epoxy needs its own hardener and a separate routine. See vinyl ester vs epoxy.
What this guide does not decide
This guide does not decide whether a resin will survive a particular chemical, concentration or temperature. That comes from the supplier's data, your design and a trial on your own process. TDS values are typical values, not batch guarantees. Mixing ratios and catalyst amounts are not given here; confirm them against the TDS and with us.
What to confirm before you order
- The grade name and catalogue code, exactly as on the data sheet.
- Quantity and unit, packaging, the delivery city or district, lead time and availability — all confirmed on enquiry.
- The current TDS/SDS revision for the batch you will receive. See how to read a TDS and SDS.
- Anything else needed for the same job: MEKP or epoxy hardener, accelerator, glass fibre mat.
The full list of what to send is in requesting a resin quotation in Nepal.
Ask for a quotation
Samrat FRP Traders is a trader, supplier, importer and reseller based in Biratnagar. The resin grades are made by Samrat Poly Resins, India. Prices are on request.