Coil Coating

Coil coating is how most painted metal roofing gets its colour. A coil of metallic-coated steel or aluminium strip is unrolled, run through a series of cleaning, treatment, painting and curing stations at high speed, and rewound as a prepainted coil ready for slitting and roll-forming. Because the paint is applied and cured under controlled conditions, the finish is far more uniform and durable than site painting.

Typical line sequence

1. Uncoil the strip and stitch it to the end of the previous coil.
2. Clean and brush the surface.
3. Apply a chemical conversion pretreatment, then dry.
4. Apply primer to one or both sides.
5. Cure in an oven, typically for 15 to 60 seconds, then cool.
6. Apply topcoat (and backer coat) and cure again.
7. Cool and rewind.

Accumulators at the entry and exit let the line keep running while coils are changed. Lines handle strip roughly 18 to 60 in (46 to 152 cm) wide at about 100 to 700 ft per minute (0.5 to 3.6 m/s). Steel coils can weigh around 25 tons.

Coatings applied

Polyesters, PVDF, plastisols, polyurethanes and epoxies are all applied on coil lines, as are laminated plastic films. The coil stock itself usually already carries a metallic coating such as galvanizing or 55% aluminium-zinc from an upstream line.

History and industry

The process dates from the 1930s; the venetian blind industry was an early user. Output is several million tonnes a year in both North America and Europe. Industry bodies include the National Coil Coating Association (NCCA) in the United States and the European Coil Coating Association (ECCA). European prepainted steel is covered by EN 10169.

Why it matters for roofs

The quality of pretreatment and primer, and the type of topcoat, govern adhesion, edge corrosion and colour retention. Roll-forming after painting requires flexible coatings, so coil paints are formulated to survive bending without cracking.