Fibre Reinforced Facade Encyclopedia: Composition
A gel coat is a pigmented, highly filled resin layer applied to the mould face before laminating, so that it cures as the outermost 0.4 to 0.8 mm of the finished fibre reinforced polymer panel. It is not a paint applied afterwards. The colour, gloss and texture of the panel are taken directly from the mould through the gel coat, and the gel coat is also the panel’s first line of defence against ultraviolet light and water.
A gel coat is applied at roughly 500 to 600 microns wet to cure at 400 to 800 microns. A two coat PVDF paint system on the same panel would be 25 to 35 microns dry. The gel coat is thick enough to be cut back and polished; a paint film is not.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Cured film thickness | 400 to 800 microns | Under 400 microns risks fibre print and undercure |
| Wet application rate | about 500 to 600 g/m² | Sprayed in two passes at right angles |
| Resin chemistry | Isophthalic NPG polyester, or vinyl ester | Vinyl ester for higher water and chemical resistance |
| Barcol hardness at demould | 35 to 45 | Below this the gel coat is undercured and will print |
| Gloss at 60 degrees | Typically 60 to 90 gloss units | Matt and satin mould surfaces give lower readings |
| Colour tolerance | Assessed against an approved range sample | Write a delta E limit into the specification |
| Service life before recoat | Typically 10 to 20 years exterior | Longer with a compatible topcoat or regular cleaning |
| Contribution to fire | Combustible; part of the tested build-up | ShapeShell-RT: AS 5637 Group 1, AWTA Product Testing |
How it is applied
The mould is cleaned, sealed and waxed or treated with a semi-permanent release. Gel coat is then sprayed onto the mould face in two crossed passes to an even wet film, checked with a wet film comb, and left to reach a firm but tacky cure. Laminating starts within the resin’s open time so the backing laminate bonds chemically to the still-reactive gel coat rather than mechanically to a fully cured film.
Behind the gel coat, good practice is a resin rich barrier layer, often a surface veil or a fine chopped strand, before the structural plies go on. That layer keeps coarse glass away from the surface and is the single most effective control for print-through. The hand lay-up and laminate entries cover what happens after that point.
Resin chemistries
- Orthophthalic polyester: lowest cost, adequate for interior and short-life work, poorest water resistance
- Isophthalic NPG polyester: the standard architectural gel coat, good gloss retention and hydrolysis resistance
- Vinyl ester: better water, chemical and blister resistance, used for wet exposure and high performance work
- Urethane modified and acrylic modified grades: improved toughness and colour stability at higher cost
Pigment chemistry matters as much as resin chemistry. Inorganic pigments hold colour under ultraviolet exposure where many organic pigments fade, which is why strong reds, oranges and bright blues are the colours most likely to shift over a facade’s life. Dark colours also run hotter, and surface temperature drives both colour change and thermal movement.
Common defects and their causes
Pinholing comes from air entrapment or solvent in the film and appears as fine holes in the cured face. Alligatoring or wrinkling is undercured gel coat attacked by the styrene in the laminating resin behind it, almost always the result of laminating too early or too thin a film. Fibre print-through is glass telegraphing to the surface as the laminate cures and shrinks, worse with thin gel coat, no barrier layer and high exotherm. Pre-release is gel coat lifting off the mould before lamination, usually a release or film thickness fault.
Blistering and osmosis show up years later on panels in long wet exposure and are driven by water permeating a thin or porous gel coat and finding soluble material behind it. Vinyl ester gel coats and a resin rich barrier layer are the usual answers. None of these defects is repairable to an invisible standard on a large flat panel, so the control is at the mould, not the site.
What a gel coat does not do
A gel coat does not change the reaction to fire of the panel by itself, and it does not make an FRP panel non-combustible. Fibre reinforced polymer has an organic matrix; the classification is earned by testing the finished build-up, gel coat included. ShapeShell-RT holds an AS 5637 Group 1 classification from AWTA Product Testing and passed the full scale BS 8414 and AS 5113 facade test at BRE Global, and those results apply to the tested make-up rather than to the resin or the coating separately.
Specifying and maintaining it
Specify the resin type, the minimum cured film thickness, the colour against an approved range sample rather than a single chip, the gloss level, and the acceptance viewing distance and lighting. Require a signed off mock-up panel as the benchmark for colour and finish, and state that colour is judged against the approved range under diffuse daylight.
In service, gel coat is cleaned with mild detergent and water. Chalking is polished back and, where a panel has weathered past that point, the face can be cut, compounded and sealed, or overcoated with a compatible air-dry fluoropolymer system. That is a genuine advantage over a factory baked coating, which cannot be restored on site to the same standard.
Standards and references
- ISO 12466 and ASTM D2583 (Barcol hardness of cured resin surfaces)
- ASTM D2244 Calculation of Colour Tolerances and Colour Differences
- ASTM D4214 Evaluating Degree of Chalking of Exterior Paint Films
- ASTM D523 Standard Test Method for Specular Gloss
- ISO 4892-2 and ASTM G155 (xenon arc accelerated weathering)
- AS 5637.1 group number and AS 1530.3 early fire hazard, applied to the finished laminate
Where this matters on a ShapeShift project
Spanda, Perth: A large public artwork where the moulded surface is the finished surface, and the gel coat carries the colour and gloss of the piece.
Related terms
Questions specifiers ask
Is a gel coat the same as paint?
No. A gel coat is a resin layer cast against the mould and cured as part of the panel, typically 400 to 800 microns thick. Paint is applied to a finished surface at 25 to 40 microns. The gel coat is integral to the laminate; paint sits on top of it.
Can a gel coat be repaired on site?
Small chips and scratches can be filled with matching gel coat, cut back and polished, and the repair is usually acceptable at normal viewing distance. Large area colour matching on a weathered panel is difficult, so damaged panels are often better replaced or fully overcoated.
How long does a gel coat last?
Ten to twenty years of exterior service before the face needs polishing or overcoating is typical, depending on colour, orientation and cleaning regime. The panel itself lasts far longer; the gel coat is the sacrificial layer.
Does a gel coat give a fire rating?
No. Reaction to fire belongs to the tested build-up, and every claim has to name the standard and the testing body. A gel coat is an organic coating and contributes fuel; it never makes a panel non-combustible.
Can a gel coat give a matt finish?
Yes. Gloss level is controlled by the mould surface and by the gel coat formulation, so matt, satin and high gloss are all available. Matt finishes hide surface waviness but hold dirt more readily.
Need a moulded colour and gloss held across a whole elevation? ShapeShell-RT panels are finished from the mould face with a pigmented gel coat. See ShapeShell-RT specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.