How to Select a Gelcoat: Five Questions That Settle It

Start with the part, not the product list

Gelcoat selection goes wrong most often when it starts from a catalogue rather than from the component. The question is not which gelcoat is best. It is what this particular surface has to survive, for how long, and what is going on behind it.

Five questions settle most cases. What is the part. What is the surface exposed to in service. Is a laminate going over this layer. How is it being applied. And does the colour have to match something that already exists. Work through those and the specification usually writes itself.

Question one and two: the part and its exposure

The exposure does most of the work in the decision. A moulding that sits indoors under shelter is asking for a cosmetic surface that stays clean and looks right. A hull that stays in the water, a pool, a water slide, a bathware moulding or an external panel in full sun for years is asking for something that holds gloss and resists water over a long service life.

That is the line between the general-purpose route and the isophthalic route. It is also worth separating weather exposure from immersion: resisting years of sunlight and resisting continuous water contact are related but not identical requirements, and a grade positioned for one is not automatically right for the other.

If the part is covered by a customer specification or a standard, that overrides judgement. Read what it names, and ask for that.

Question three: gelcoat or topcoat

This single question causes more avoidable rework than any other in the category. A gelcoat is formulated to stay tacky on its air side so the laminate applied over it bonds properly. A topcoat has wax in it so that it cures tack-free as a final exposed surface.

So the test is simple: is anything going on top of this layer. If yes, it is a gelcoat. If the layer you are applying is the last one and will be exposed, it is a topcoat you want. Buying a topcoat and laminating over it, or buying a gelcoat and leaving it as the final surface, both produce predictable problems.

Question four and five: application and colour

Application method is a genuine selection input rather than an afterthought. Brush and roller work is possible, but spraying is what suppliers and fabricators generally recommend for finish quality and, more importantly, for an even film. Since uneven or excessive thickness is behind cracking, sagging and pre-release, the way the gelcoat goes on affects the defect rate directly.

Colour is the question people raise too late. Pigment dispersion, mixing practice and batch consistency all influence how well one run matches another, and matching an existing part on site is a harder request than choosing a standard colour for new work. Raise it before the production run, order what you need for the job together where that is possible, and keep the batch references with the job.

Where a shade has to match panels already installed, ask to check against a sample rather than against a colour name.

What to confirm before committing a run

Before a production quantity is committed, it is worth having four things settled: the grade and its resin type, confirmed from the manufacturer’s document rather than from a product name; the film thickness and cure guidance the manufacturer publishes for that grade; the safety documentation your workshop needs in order to write a handling procedure; and the colour and batch arrangement for the quantity you are taking.

Samrat FRP Traders supplies and imports gelcoat for Nepal from Samrat Poly Resins, India. We are a trader, supplier, importer and reseller, and we do not manufacture, formulate, test or approve gelcoat for an application.

GP Gelcoat Resin has its supplier Technical Data Sheet and Safety Data Sheet on file, and ISO Gelcoat has its supplier Technical Data Sheet on file. For either grade we publish only what the manufacturer’s sheet states, and put anything it does not cover to the supplier.

Frequently asked questions