By Hu Xianzhe, Cold Roll Forming Technical Engineer at ZTRFM | Last Updated: September 15, 2026
A customer in coastal Vietnam complained that his PPGI roofing sheets started fading after 8 months. He'd ordered "standard" PPGI from a trading company without specifying the coating system. The paint was PE (polyester), the zinc coating was Z80 (80g/m²), and the primer was minimal. In a coastal environment with salt spray and UV, that combination won't last 2 years.
I'm Hu Xianzhe, Cold Roll Forming Technical Engineer at ZTRFM. I source prepainted steel coil for our platform's verified suppliers and help buyers specify the right material for their environment. The difference between PPGI, PPGL, and AZ150 isn't academic — it determines whether your roof lasts 5 years or 25.
PPGI: Prepainted Galvanized Iron
PPGI is the most common prepainted steel. The base metal is hot-dip galvanized steel (zinc coating), then a primer and topcoat paint are applied in a continuous coil coating line.
Coating structure (outside to inside):
- Topcoat: 15-25 microns (usually PE polyester, or SMP silicon-modified polyester for better UV resistance)
- Primer: 5-8 microns (epoxy or polyurethane, provides corrosion protection and paint adhesion)
- Zinc coating: Z80 to Z275 (80-275 g/m² total both sides)
- Base steel: usually SGC340, SGC400, or SGC440 (structural grades)
- Backer coat: 5-10 microns (simpler paint, protects the zinc from corrosion from underneath)
Zinc coating levels:
- Z80 (80g/m²): Indoor use or dry climates only. Not recommended for roofing.
- Z120 (120g/m²): Basic outdoor use, temperate climate, no salt spray. Minimum I'd recommend for roofing.
- Z180 (180g/m²): Good outdoor durability, most common for general roofing.
- Z275 (275g/m²): Heavy-duty outdoor, industrial environments, coastal areas within 5km of shore.
Cost: PPGI is the cheapest prepainted option. The PE topcoat version is what most budget roofing sheets use.
Lifespan (PE topcoat, Z120): 5-8 years in temperate climate, 2-4 years in coastal or tropical.
Lifespan (SMP topcoat, Z275): 15-20 years in temperate, 8-12 years in coastal.
PPGL: Prepainted Galvalume (Aluzinc)
PPGL uses a different metallic coating: aluminum-zinc alloy (55% aluminum, 43.5% zinc, 1.5% silicon) instead of pure zinc. This alloy is known as Galvalume (US), Aluzinc (Europe), or Zincalume (Australia).
The aluminum provides a physical barrier (it forms a stable oxide layer), while the zinc provides sacrificial (cathodic) protection at cut edges. The combination is more corrosion-resistant than pure zinc at the same coating weight.
Coating weight designation:
- AZ100 (100g/m²): Equivalent corrosion resistance to approximately Z180 galvanized.
- AZ150 (150g/m²): Equivalent to approximately Z275 galvanized. The most common PPGL spec for roofing.
- AZ165 (165g/m²): Heavy-duty, for severe environments.
Key advantages over PPGI:
- 2-3x better corrosion resistance at equivalent coating weight
- Better heat reflectivity (aluminum reflects radiant heat, reducing attic temperatures by 5-8°C)
- Smoother surface finish (better paint adhesion, smoother final product)
- Resists corrosion at cut edges better (the aluminum barrier protects exposed steel)
Cost: PPGL is typically 8-15% more expensive than PPGI at equivalent specs.
Lifespan (SMP topcoat, AZ150): 25-35 years in temperate, 15-20 years in coastal.
AZ150 vs Z275: Which Is Better?
This is the question I get most often. AZ150 and Z275 are the two "heavy-duty" options, and they cost roughly the same. Here's my decision framework:
| Factor | Z275 (PPGI) | AZ150 (PPGL) |
|---|---|---|
| Corrosion resistance | Good | Better (2-3x at cut edges) |
| Heat reflectivity | Standard | Higher (cooler roof) |
| Surface smoothness | Slightly grainy | Smoother |
| Paint adhesion | Good | Better (smoother surface) |
| Sacrificial protection at cut edges | Strong (pure zinc) | Moderate (aluminum barrier + zinc) |
| Cost | Baseline | +8-15% |
My rule of thumb: for coastal or tropical environments, go AZ150. For dry or temperate climates where cost matters, Z275 is fine. For industrial areas with acid rain or chemical exposure, AZ150 is the safer choice because the aluminum barrier resists chemical attack better.
Paint Systems: PE vs SMP vs PVDF
The metallic coating protects the steel. The paint protects the metallic coating. Don't conflate the two.
| Paint Type | UV Resistance | Color Retention | Cost Premium | Best For |
|---|---|---|---|---|
| PE (Polyester) | Basic | 3-5 years | Baseline | Budget projects, indoor, temporary structures |
| SMP (Silicon-Modified Polyester) | Good | 8-12 years | +10-15% | General outdoor roofing, most common choice |
| PVDF (Fluorocarbon) | Excellent | 15-25 years | +40-60% | High-end projects, coastal, desert, landmark buildings |
If a supplier quotes you "PPGI" without specifying the paint type, it's PE. That's the default. For roofing in any sunny climate, specify SMP minimum. PVDF is worth it for premium projects or severe UV environments (Middle East, Australia, high-altitude regions).
Quick Q&A
How do I verify the coating weight on received coil? Request a mill test certificate (MTC) with each coil shipment. The MTC should state the coating weight (g/m²), paint type, paint thickness, and base metal grade. For independent verification, a lab can do a strip-and-weigh test (ASTM A90).
Can I form PPGL on the same machine as PPGI? Yes. The mechanical properties are similar. But PPGL is slightly harder on rollers (the aluminum-zinc surface is more abrasive). Chrome-plated rollers are recommended.
What's the minimum order quantity for prepainted coil? Typically 25 tons (one coil). Some suppliers will do 5-ton minimums at a premium. Coil width is usually 1000mm, 1219mm, or 1250mm.
How should I store prepainted coil? Keep it dry, indoors, away from direct sunlight. Condensation between coil wraps causes white rust (zinc oxidation). If coil gets wet, unroll and dry it immediately. Don't store coil on bare ground.
Frequently Asked Questions (FAQ)
How can coating weight be verified on received coil?
Request a mill test certificate stating coating weight, paint type, paint thickness and base-metal grade. For independent verification, the article recommends a laboratory strip-and-weigh test under ASTM A90.
Can PPGL be formed on the same machine as PPGI?
Yes. Their mechanical properties are similar, although the aluminum-zinc surface of PPGL is more abrasive, so chrome-plated rollers are recommended.
What is the minimum order quantity for prepainted coil?
The article gives a typical minimum of 25 tons, with some suppliers offering 5-ton minimums at a premium. Common widths are 1000 mm, 1219 mm and 1250 mm.
How should prepainted coil be stored?
Store it dry and indoors, away from direct sunlight and off bare ground. If the coil becomes wet, unroll and dry it immediately to prevent white rust between wraps.
