What Color Coated Galvanized Steel Coil Is
Color coated galvanized steel coil, often called prepainted coil, is produced by applying an organic paint system to a continuously hot-dip galvanized substrate. When the substrate carries a pure zinc coating the product is known as PPGI or GI coil; when the substrate carries a 55 percent aluminium-zinc coating the product is called PPGL. Both routes start from cold rolled steel for a smooth, uniform surface.
Substrate: cold rolled steel, typically DX51D, SGCC, SPCC or DC01
Metallic coating: zinc from Z100 to Z275, or aluminium-zinc around 150 g/m²
Paint build: primer plus topcoat, usually 20 to 30 µm total dry film
Finished form: coil for profiling, or cut sheet for flat panels
Base Grades Compared: DX51D, DC01, SPCC and SGCC
The four designations in the product name come from three different standards, and they are not interchangeable. Two of them describe coated forming grades and two describe uncoated cold rolled grades that require a coating before they can be used outdoors.
| Grade | Standard | Product type | Typical tensile range | Usual role |
|---|---|---|---|---|
| DX51D | BS EN 10346 | Hot-dip zinc coated, forming grade | 270 to 500 MPa | Roofing, cladding, ducting |
| SGCC | JIS G 3302 | Hot-dip zinc coated, commercial quality | 270 to 500 MPa | Roofing and general cladding |
| SPCC | JIS G 3141 | Cold reduced, uncoated | 270 to 410 MPa | Formed parts, needs coating |
| DC01 | EN 10130 | Cold rolled, uncoated | 270 to 410 MPa | Panels, ducting, furniture |
DC01 in EN 10130 is specified with a minimum yield strength of 140 MPa and a tensile range of 270 to 410 MPa, which makes it easy to bend and draw but unsuitable for load bearing profiles. Deeper drawing versions such as DC03 and DC04 extend the forming range further. For roofing sheet the coated forming grades DX51D and SGCC remain the practical choice, because the zinc layer protects the steel at cut edges and fixing holes where paint alone cannot help.
Metallic Coating Options on the Substrate
Coating mass drives service life, and the same conversion used for bare galvanized coil applies here: the designation states grams of zinc per square metre over both faces.
| Designation | Total coating mass | Thickness per side | Recommended environment |
|---|---|---|---|
| Z100 | 100 g/m² | about 7 µm | Dry interior |
| Z180 | 180 g/m² | about 12.6 µm | Rural and urban exterior |
| Z275 | 275 g/m² | about 19 µm | Roofing, cladding, general industry |
| AZ150 | 150 g/m² aluminium-zinc | about 20 µm | Coastal and high temperature service |
Aluminium-zinc coatings resist heat and cut edge corrosion better than pure zinc in coastal air, while pure zinc offers stronger sacrificial protection around scratches. The choice should follow the corrosivity category of the site and the expected maintenance interval rather than price alone.
Paint System Selection for Roofing
The topcoat determines colour retention and gloss retention. Polyester systems are the workhorse for general roofing, silicone modified polyester systems tolerate higher service temperatures, and fluorocarbon systems are specified where appearance must hold for decades on prestige facades. Epoxy primers bond well to zinc and act as a barrier between metal and topcoat.
Primer dry film 5 to 7 µm, applied first on the pre-treated surface
Topcoat dry film 18 to 25 µm, colour matched to a master panel
Back coat 5 to 10 µm, protecting the reverse face during service
Curing peak metal temperature set by the paint system, not by line convenience
Film thickness must be measured on the finished sheet, and the readings should be taken on the flat areas rather than on the ribs, where the coating is stretched during profiling.
Roofing Profiles and Mechanical Requirements
Roofing sheets are roll formed from coil into corrugated or trapezoidal profiles. The base metal thickness usually lies between 0.30 mm and 0.80 mm, and the profile depth ranges from about 15 mm for a shallow corrugated sheet to 40 mm or more for a structural trapezoidal deck. Deeper profiles require a higher yield strength and a lower bend radius, which is why structural grades are often specified for long span roofs.
During profiling the paint is stretched over the crests of the ribs, so coating flexibility matters as much as adhesion. Tensile bend tests and impact tests on the finished profile reveal whether the chosen paint and coating combination can survive the forming operation without cracking.
Quality Checks and an Ordering Checklist
Release testing for color coated galvanized coil normally includes coating mass, paint adhesion after bending, impact resistance, pencil hardness, gloss and colour difference, plus salt spray exposure to ISO 9227 for a defined number of hours. Buyers should confirm the following points before the coil is produced.
Base grade and its controlling standard, for example DX51D to BS EN 10346
Zinc or aluminium-zinc coating mass and the minimum single spot value
Paint system, colour reference and required dry film thickness
Coil dimensions, inner diameter, coil weight and edge condition
Forming limits such as minimum bend radius and profile depth
A written technical agreement that fixes these parameters prevents the most common complaint in this trade, which is a colour or thickness mismatch between batches delivered months apart.
Frequently Asked Questions
Q: Is PPGI the same as color coated galvanized coil?
Yes. PPGI describes prepainted galvanized iron, a zinc coated substrate with an organic paint system. PPGL is the equivalent product on an aluminium-zinc substrate.
Q: Can DC01 or SPCC be used directly for roofing?
Not outdoors. Both are uncoated cold rolled grades from EN 10130 and JIS G 3141, so they rust quickly when wet. They must be coated or painted before external service.
Q: Which coating should be used near the sea?
A zinc coating of Z275 or heavier, or an aluminium-zinc coating such as AZ150, combined with a thicker topcoat and edge protection at all fixings.
Q: Why does the same colour look different on two deliveries?
Small differences in dry film thickness, curing temperature or pigment batch shift the reflected light. Colour should therefore be judged against a retained master panel, not from memory.
Q: What thickness of steel is normal for roofing sheet?
Most profiles run from 0.30 mm to 0.80 mm base metal thickness. Long spans and high wind loads push the specification towards the upper end of that range.
Q: How long does color coated coil last?
Service life depends on the coating mass, the paint system and the corrosivity of the site. A Z275 substrate with a good topcoat in a rural atmosphere typically performs far longer than a thin Z100 product in coastal air.