Ultraviolet does not cause corrosion. It destroys what prevents it. That distinction sounds like semantics until you see a site where the coating was specified entirely by the book — right environment category, right thickness — and three years on it crumbles to chalk under your finger. This article looks at what sunlight actually does to a coating, why it matters more in our region than in temperate climates, and where corners are most often cut.

What physically happens to a coating in the sun

Any paint coating is pigment held by a binder. The ultraviolet part of the solar spectrum carries enough energy to break chemical bonds in the binder molecules. The process is called photodegradation and proceeds in stages:

Loss of gloss. The first sign, visible within the first year or two. The surface dulls.
Chalking. The binder in the top layer breaks down and the pigment is left loose on the surface. Run your hand over it and coloured powder comes away. That powder is destroyed coating.
Colour change. Fading, yellowing, uneven patches — most noticeable on south and west elevations and on upward-facing surfaces.
Loss of thickness. Chalking is not cosmetic: the layer is physically carried away by rain and wind. A coating specified for 25 years reaches critical thickness far sooner.
Cracking and loss of adhesion. The final stage, after which water reaches the steel and ordinary corrosion begins — but with years already lost.

The key point: by the time the first rust appears on a structure, the coating has been dead for several years. Rust is not the start of the problem but a late symptom of it.

Why this bites harder here

Our climate adds three amplifying factors to the ultraviolet itself.

Number of clear days. We get substantially more hours of sunshine per year than temperate regions. The annual ultraviolet dose a coating receives is correspondingly higher — and binder ageing depends on accumulated dose, not on calendar time.

Surface temperature. Steel in the sun heats up far above air temperature — a dark structure on a summer day can be too hot to touch. Most chemical processes, binder degradation among them, accelerate as temperature rises. Ultraviolet and heat work together, not in turn.

Daily temperature swings. The steel expands by day and contracts by night, and the coating must follow the substrate. While it remains elastic this passes without consequence; as the binder ages, elasticity falls, and at some cycle micro-cracking appears.

Hence a practical conclusion that is often ignored: a coating system that achieved its design life in a temperate climate may not reach half of it here. The corrosivity category will be the same — it does not account for ultraviolet.

How different materials handle sunlight

Ultraviolet resistance is determined by the binder type, and the spread is wide.

Epoxy coatings are not UV resistant. This is a known and acknowledged property, not a defect. Epoxy is excellent as a primer and as an intermediate coat: good adhesion to steel, high chemical resistance, builds thickness. But outdoors it chalks.
Polyurethane coatings are UV resistant. That is precisely why they are used as topcoats: they hold gloss and colour and protect the epoxy beneath.
Acrylic coatings are UV resistant and hold colour well, but generally give way to polyurethanes on mechanical and chemical resistance.
Alkyd coatings sit in between and are rarely used today in demanding systems on steel.

Hence the classic three-coat scheme for external structures: primer — epoxy intermediate coat — polyurethane topcoat. Each coat does what it does best. The topcoat here is not decoration but a functional part of the system: it is consumed first, taking the ultraviolet on itself.

Four ways to save money and lose more

1. Omit the topcoat

The most common and the most expensive mistake. The logic sounds convincing: “epoxy is tougher than polyurethane, why put anything over it”. It is tougher — mechanically and chemically. But not in sunlight. Saving one coat removes the protection from the other two and produces chalking within the first years.

2. Apply the topcoat thinner than specified

The topcoat is consumed in service — that is normal, that is its job. But it should be the topcoat that is consumed, not the layer beneath. A shortfall in topcoat thickness means that within a few years ultraviolet reaches the epoxy intermediate coat, and from then on degradation runs through the whole system.

3. Choose a cheap pigment

Colour stability depends not only on the binder but on the pigment. Some pigments fade markedly faster than others, and this applies especially to bright colours — red, yellow, orange. For corporate colours that must look identical across every site, this is a question not only of protection but of how the company looks.

4. Ignore the colour

Dark surfaces run hotter than light ones. On exposed structures in our climate, choosing a dark colour means voluntarily raising the working temperature of the coating, and with it the rate at which it ages. Where colour is not dictated by client requirements or by marking, light shades are objectively the better buy.

What this means in money

The topcoat is the only part of the system that can be renewed without removing the whole coating. If the system is sound and only the topcoat is worn, it is overcoated: washed, lightly abraded and given a fresh coat. That is incomparably cheaper than a full rebuild, because it does not require blast cleaning back to steel.

But the window for that is limited. It is open only while the damage is confined to the topcoat. Once ultraviolet has reached the intermediate coat and delamination has begun, there is only one option left — strip everything back to steel and start again, with scaffolding and a production shutdown.

Hence a rule worth building into the maintenance budget: external coatings should be inspected and the topcoat renewed on condition, not when rust appears. The first signs are loss of gloss and chalking — and that is exactly when renewal is cheap.

What belongs in the specification

To avoid arguing about this after the fact, a specification for coating external structures needs four items:

Binder type for the topcoat — stated explicitly, not “enamel for steel” but a specific class with proven ultraviolet resistance.
Topcoat thickness — as a separate line, not only the total system thickness.
Colour stability requirement, where appearance matters.
Inspection intervals and the criterion on which the decision to renew the topcoat is taken.

In brief

The sun destroys not the steel but the coating — and it does so quietly, with no visible consequences, until it is too late. In our climate the ultraviolet dose and the surface temperature are higher than in temperate latitudes, so systems brought from there live shorter lives than their data sheets promise.

Protection against this is simple and inexpensive if considered in advance: a UV-resistant topcoat at the specified thickness, sensible colour selection, and inspections before rust appears rather than after. Saving on the topcoat does not reduce the cost of protection — it converts a cheap overcoat into an expensive full rebuild.

The next article in this series covers the most specialised case of all: tanks for aviation fuel storage, from material selection to acceptance of the finished lining.

Why coatings fail early: ultraviolet and cutting corners on the topcoat