Aluminium precoated coil has been widely used in fabricated letters because it's lightweight, easy to bend, and available in long strips. But in real-world use — especially outdoors — it has consistent failure points.
At Aussie 3D Fabrications, we've seen these failures firsthand. Here's what we've learned about why aluminium coil is risky for exterior signage.
1. Coating Systems on Aluminium Coil
Precoated aluminium coil is typically finished with an acrylic or polyester-based coating system. These coatings are applied to the surface of the aluminium and cured, but they don't chemically bond into the base metal. They are dependent on adhesion strength between the paint film and the aluminium substrate.
When exposed to elevated temperatures, above 60 °C at the surface, the coating can soften. Outdoor signs often exceed these temperatures on a sunny wall, which reduces bond strength and makes the coating vulnerable to release.
2. Thermal Expansion Mismatch
Aluminium has a relatively high coefficient of thermal expansion. The coating materials used on precoated coil (often acrylic- or polyester-based) expand and contract at different rates than the aluminium beneath.
In outdoor environments, the daily cycle of heating and cooling causes stress at the coating–metal interface. Over time, this leads to:
- Loss of adhesion,
- Peeling or blistering,
- Complete coating release.
This isn't a matter of "bad glue" or lack of reinforcement. It's a fundamental mismatch between substrate and coating under repeated heat cycling.
Thankfully Colorbond steel has that more durable coating, which allows glues to do their job more effectively, with out the risk of coating failure.
3. Indoor vs Outdoor Use
Indoors, aluminium coil performs acceptably. Temperatures remain stable, UV exposure is minimal, and the expansion mismatch does not cause rapid degradation. That's why coil is common in internal signage and displays.
Outdoors, however, the combination of heat, UV, and thermal cycling accelerates failures. We've seen issues develop even on letters as small as 300 mm high within just a few months.
4. Why We Use Colorbond® Steel for Outdoor Letters
To avoid these failure points, we've switched to using Colorbond® steel for outdoor fabricated letters.
Colorbond® steel is manufactured by BlueScope using a proven multi-layer system:
- A Zincalume® steel base (aluminium/zinc/magnesium alloy coating) that provides corrosion resistance.
- A conversion layer to ensure strong adhesion of further coatings.
- A corrosion-inhibitive primer baked onto the surface.
- A durable colour topcoat, oven-cured for long-term performance.
This system has been tested for more than 50 years in Australian conditions — including UV exposure, heat, humidity, and salt spray. That's why Colorbond® is trusted for roofing and cladding across the country.
The failure points of aluminium coil are not due to workmanship — they are inherent in the way precoated aluminium behaves under outdoor conditions.
- Heat softens the coating.
- Expansion mismatch stresses the bond.
- Thermal cycling causes release over time.
That's why at Aussie 3D Fabrications we've made a clear decision:
- Aluminium coil is used indoors only.
- Outdoor fabricated letters are made exclusively with Colorbond® steel.
This ensures the signs we supply to our trade customers deliver the strength, finish, and longevity they expect.
We have spent months testing glue formulations and have found a clear winner that offers a perfect combination of temperature resistance and durability. We have full confidence in our EcoSteel system and offer a 5 year warranty on all our outdoor fabricated letters.