Short answer
On most glass-reinforced plastic the colour is in the gelcoat: a pigmented layer of resin put into the mould first, so that it becomes the outer face of the finished part. It is part of the moulding, not a coat added afterwards. Outdoors that resin oxidises under ultraviolet, and in exposure tests it yellows before it chalks and loses its gloss. How quickly depends on the resin, its stabilisers, the pigment and the climate, and laboratory tests predict it poorly.
Relative to other materials in this section, not a measurement of a particular product.
| Process | What you see | Driven by | Slowing it |
|---|---|---|---|
| Gelcoat yellowing | Whites and pale colours turn cream or yellow, before the gloss goes | Ultraviolet-driven oxidation of the resin at the surface | Gelcoats formulated with ultraviolet absorbers; covers and shade |
| Gelcoat chalking and gloss loss | Dull, powdery surface on which colours look paler | Continued surface breakdown under sunlight, oxygen and moisture | Stabilised exterior-grade gelcoat; keep the surface clean and protected |
A gelcoat is a modified resin — most often an unsaturated polyester, sometimes an epoxy — applied as a liquid to the inside of the mould before any reinforcement. It cures to a cross-linked skin and is then backed with the structural laminate of resin and glass or carbon fibre. When the part is released, the face that was against the mould is the gelcoat, usually pigmented to give a coloured, glossy surface. On boats it is typically 0.5 to 0.8 mm thick, and it carries a thixotropic additive so that it stays on the vertical parts of the mould while it cures. The colour is therefore neither in the body of the laminate nor painted on afterwards: it is one moulded layer.
Because the gelcoat sets against the mould, its surface takes its smoothness from the mould, and a part made this way comes out with a glossy face. In light, new gelcoat behaves as the gloss rows of the table on this page describe — deep colour away from reflections, an image of the sun or the sky at the mirror angle — which is why a dark gloss hull shows reflections so strongly. It also means the colour is judged through the top of a resin layer, so anything that roughens or yellows that layer changes the colour seen, whether or not the pigment below has altered.
Polyester resin degrades outdoors under sunlight, oxygen, moisture and heat acting together, and the damage begins at the surface. A trade-journal study that exposed polyester gelcoats outdoors, under concentrated sunlight and in two kinds of laboratory cabinet found the same order each time: colour change — yellowing — appeared first, and chalking and loss of gloss followed later. It also found that ultraviolet absorbers reduced the yellowing, whereas a hindered-amine stabiliser used alone did not. On a white or pale part the yellowing is what an owner notices; on a dark one it is the later chalk and dullness, which lay a pale veil over the colour. The failure table on this page gives the test that separates each.
The same study compared one year outdoors with fluorescent-ultraviolet and xenon-arc cabinets and with concentrated natural sunlight, and concluded that none of the laboratory methods matched the outdoor result: the fluorescent-ultraviolet cabinet produced a dramatic colour change that had no counterpart outdoors, and the number of cabinet hours equivalent to a year outdoors was different for yellowing and for gloss loss. That echoes findings for paint, where some standard accelerated tests exaggerate damage. For anyone choosing or matching a gelcoat colour the consequence is practical. A quoted number of test hours ranks products against each other; it does not say how many seasons a hull or a cladding panel will hold its colour in a given climate.
| Surface and finish | Light | Viewed | What changes | What to do | Evidence |
|---|---|---|---|---|---|
| Dark gloss paint, lacquer, laminate or moulded plastic (gloss) | Single small source (spotlight, downlight, bare lamp) | At the mirror angle of the light | A bright image of the lamp replaces the colour at that spot; the darker the colour, the stronger the contrast between the hot spot and the surface around it. | Move or diffuse the lamps so their mirror angle misses the usual viewpoint, or choose a satin or matt finish for dark colours under spotlights. | Strong |
| Dark gloss paint, lacquer, laminate or moulded plastic (gloss) | Single small source (spotlight, downlight, bare lamp) | Face-on, away from any reflection | Away from the reflection the colour looks darker and more saturated than the same colour in a matt finish. | Compare samples in the finish that will be used, held at the same angle and away from reflections. | Strong |
| Gloss or semi-gloss paint, lacquer or laminate (gloss) | Diffuse or large-area light (overcast sky, big window, lit ceiling) | From an angle | A large pale reflection of the window, sky or lit ceiling lies over part of the surface, and the colour there looks lighter and washed out. | Keep gloss for surfaces that do not face large windows at the mirror angle, or lower the sheen; blinds change the reflection, not the paint. | Moderate |
| What goes wrong | How to recognise it | Can it be reversed? | What to do |
|---|---|---|---|
| Yellowing | The shift is towards yellow or brown and is strongest in whites and clear layers. Note where it is worst: on the sunlit face, or — for oil and alkyd paint — in the places light never reaches. | Partly reversible. Whether the yellow is in a layer that can be removed or renewed — a varnish, a coat of paint — or in the object's own substance, as with paper, plastic and silk, where it stays. | Repaint or recoat a yellowed finish; Leave removal of a yellowed varnish on a painting to a conservator; Replace yellowed plastic parts; surface treatments do not restore the material |
| Gloss loss and burnishing | View the surface along its length against the light: the change is in how it reflects, not in its colour face-on. Dark colours show it most. | Partly reversible. Only the surface is affected, so a new coat restores it; but a burnished patch cannot be made matt again, and touching up a patch rarely matches the sheen around it. | Repaint the whole wall section or panel, not the patch; Clean an exterior coating first: part of the dullness may be dirt and chalk |
| Chalking | Rub a fingertip or a dark cloth across the surface: chalk transfers. This is how the standard test methods assess it. | Partly reversible. The loose powder can be cleaned off, which recovers some colour, but the binder that has eroded is gone and the surface will chalk again. | Wash off the chalk to see what colour remains; Clean the chalk off before recoating |
| Ultraviolet damage to the material | A change of texture or strength as well as of colour, confined to the exposed face, on materials known to absorb ultraviolet. | Irreversible. Broken polymer and fibre cannot be mended. A damaged surface layer can sometimes be removed or recoated, which is replacement, not repair. | Recoat or refinish once the degraded layer has been removed; Replace embrittled parts |
Why: Ultraviolet-driven oxidation of the resin at the gelcoat surface.
Fix: Compare with a shaded area to confirm; the yellow is in the resin itself, so cleaning will not remove it.
Why: The surface has chalked and lost gloss, veiling the pigment.
Fix: Wash and wet a small area: if the colour returns while wet, the pigment is intact beneath.
Why: New gelcoat was matched to the original reference, not to the aged surface.
Fix: Match to the part as it is now, and expect the two to diverge again as they age.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactModerate evidence
Gelcoat is a modified polyester or epoxy resin applied to the mould in the liquid state and backed with the composite laminate, so that it forms the visible surface; it is usually pigmented and on marine craft is typically 0.5 to 0.8 mm thick.
Caveat: A sparsely cited reference article; the thickness is given for boats.
Source: Gelcoat
FactModerate evidence
In outdoor and accelerated exposure of polyester gelcoats, colour change (yellowing) occurred before chalking and loss of gloss, and ultraviolet absorbers reduced the yellowing while a hindered-amine light stabiliser alone did not.
Caveat: One trade-journal study of one resin family, published in 2002; the sequence is used, not its figures.
Source: UV Inhibitors in Polyester Gelcoats (PCI Magazine, September 2002)
FactModerate evidence
None of the laboratory weathering methods tested on polyester gelcoats matched the results of real outdoor exposure, and the exposure time equivalent to one outdoor year differed between colour change and gloss loss.
Caveat: The same single study; its equivalence figures are not reproduced.
Source: UV Inhibitors in Polyester Gelcoats (PCI Magazine, September 2002)
FactStrong evidence
Polymeric materials degrade outdoors through the combined action of sunlight, air, heat and moisture, with fading, chalking, surface cracking and yellowing among the results.
Colourwise interpretationLimited evidence
A gelcoat repair matched to the original colour reference will usually show against an aged part, because the surrounding gelcoat has yellowed and dulled since it was made.
Based on: Follows from the weathering sequence in the cited gelcoat exposure study: the installed surface has moved away from the colour it was made to.
Caveat: Not tested by a cited source; how visible it is depends on age and exposure.
Reviewed 1 October 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.