Definition of Colour-Coated Galvanized Coil
Colour-coated galvanized coil, also called prepainted galvanized steel or coil-coated steel, is a factory-finished flat steel product. A continuously galvanized steel coil is uncoiled and passed through a coil-coating line, where the surface is cleaned and chemically treated, then given a primer and one or more topcoats, and finally cured in an oven and recoiled. The result is a steel sheet that already carries its final colour, gloss and texture, so downstream users shear, profile and bend it without any painting operation of their own. When the substrate is zinc coated, the product is often abbreviated as PPGI; when the substrate is an aluminium-zinc alloy coating, the abbreviation is PPGL.
The Layer-by-Layer Structure
Performance comes from the way the layers work together rather than from any single one of them. Each layer has a defined job, and a failure in any layer shows up as premature corrosion or paint loss.
| Layer | Material | Function | Typical value |
|---|---|---|---|
| Substrate | Cold-rolled steel coil | Strength, stiffness and formability | 0.3 mm to 1.2 mm |
| Metallic coating | Hot-dip zinc, zinc-iron alloy, electrolytic zinc, or aluminium-zinc alloy | Sacrificial anode protection plus a first physical barrier | Z60 to Z275, or AZ150 for alloy coating |
| Conversion layer | Phosphate or chrome-free passivation | Bonds the organic coating to the metal and slows under-film corrosion | Sub-micrometre film |
| Primer | Epoxy or polyester | Corrosion protection and adhesion of the topcoat | About 5 micrometres to 7 micrometres |
| Topcoat | Polyester, silicone modified polyester, high durability polyester, polyvinylidene fluoride, plastisol | Colour, gloss, texture, weather resistance and abrasion resistance | About 15 micrometres to 25 micrometres |
Characteristics and Application Areas
Main characteristics
Combined corrosion protection: the zinc layer protects the steel sacrificially at cut edges and scratches, while the organic film blocks water, oxygen and pollutants.
Decorative freedom: a wide range of colours, gloss levels, textures and printed effects is available, which suits architectural and appliance design work.
Good formability: both the substrate and the coatings are selected to survive bending, profiling and moderate drawing.
Long service life: in a suitable environment, a well-specified coating system can last from 10 to 30 years before repainting is needed.
Light weight with useful strength: high stiffness per unit mass compared with masonry or concrete, which reduces the load on the supporting structure.
Fast construction: panels are cut, profiled and fixed on site, so site labour and wet trades are largely removed.
Environmental options: chrome-free pretreatment and water-borne or low-solvent paint systems reduce the process footprint.
Key Parameters to Specify
| Parameter | Common range or option |
|---|---|
| Substrate thickness | 0.3 mm to 1.2 mm, with heavier gauges up to 1.5 mm on request |
| Coil width | 600 mm to 1500 mm, with 1000 mm, 1200 mm and 1250 mm the usual stock widths |
| Metallic coating type | Hot-dip zinc GI, zinc-iron alloy GA, electrolytic zinc EG, aluminium-zinc alloy GL |
| Metallic coating mass | Z60, Z80, Z120, Z180, Z275 as total both-side mass, or AZ150 for alloy coating |
| Topcoat resin | PE, SMP, HDP, PVDF, PVC plastisol |
| Dry film thickness | Front 20 to 25 micrometres after two coats and two bakes, back 5 to 10 micrometres |
| Coating configuration | Front and back build-up such as 2 over 1 or 2 over 2 |
| Colour and surface | Colour reference from a standard card, gloss level, smooth, grain or printed texture |
| Coil dimensions | Inner diameter 508 mm or 610 mm, with coil weight agreed per order |
Main application areas
Construction, the largest market: roofs and external walls of industrial plants and warehouses, roofs and walls of large public buildings, residential roofing, canopies, garage doors, partitions and portable cabins.
Home appliances: housings and internal structural parts of refrigerators, washing machines, air conditioners, microwave ovens and water heaters.
Transport: inner and outer panels of vehicle bodies and containers.
Furniture and office equipment: filing cabinets, office partitions, lockers and shelving.
Other uses: signboards, ventilation ducts and electrical cabinets.
How to Choose the Right Build-Up
Service environment first: coastal, industrial and humid sites need a heavier metallic coating and a more durable topcoat, while indoor parts allow a lighter build-up.
Appearance durability: colour retention and gloss retention depend mainly on the topcoat resin, with PVDF giving the best outdoor performance and PE the most economical indoor performance.
Processing: parts that are bent sharply or drawn need a ductile substrate and a coating with enough flexibility to follow the deformation.
Cost: coating type, coating mass and film thickness drive most of the price difference, so they should be matched to the actual exposure rather than maximised.
Fire requirements: some building applications call for specific reaction-to-fire performance, which constrains the coating system that can be used.
Regulatory compliance: restricted substances rules and chrome-free pretreatment requirements may limit the choice of pretreatment and pigments.
Frequently Asked Questions
Q: What is the difference between GI and GL substrates?
GI is hot-dip pure zinc coating, which protects steel sacrificially, while GL is an aluminium-zinc alloy coating with better barrier performance at high temperature and in some atmospheric conditions but different cut-edge behaviour.
Q: How thick is the paint on a typical product?
A common specification is 20 to 25 micrometres of dry film on the front face after two coats and two bakes, and 5 to 10 micrometres of backcoat on the reverse face.
Q: Can the coil be bent or profiled after coating?
Yes, that is the purpose of the product, but the bending radius, the substrate ductility and the coating flexibility must be matched, and very tight radii can crack the film.
Q: Which topcoat should be used outdoors?
PVDF gives the best colour and gloss retention in strong sunlight, HDP and SMP sit in the middle, and standard PE is normally reserved for indoor or lightly exposed parts.
Q: How long will the coating last?
In a moderate inland environment a properly specified system commonly gives 10 to 30 years of service before repainting, but life falls sharply in coastal or heavily polluted air.
Q: Why do cut edges need attention?
The organic film cannot protect a cut edge; only the metallic coating does, so a heavier zinc or alloy coating and prompt edge treatment are the practical ways to control edge rust.