Pre-painted steel is now more popular among buyers over site-painted steel. This is because it saves a lot of time and also provides consistent color and protection from day one.
However, many buyers don’t have a clear idea about the importance of the coating weight and paint systems. Another common FAQ from our customers is “When to use pre-painted galvanized steel (PPGI) or pre-painted galvalume (PPGL)?” Understanding the difference in the corrosion resistance of PPGI vs. PPGL is also a main decision criterion.
If you choose the wrong specification, your parts can face early failure even if the coating quality is at the highest level. This guide will help you understand how to select the best pre-painted steel for your application.
Two Main Pre-Painted Coating Types: PPGI and PPGL
Pre-painted steel is applied on a metallic-coated base sheet. In simple terms, the paint is not the first protection layer. The real protection starts from the metallic coating under the paint. In the market, there are two main pre-painted types based on the base coating:
- PPGI (Pre-Painted Galvanized Steel) – Uses a zinc (Z) coating on the base steel, which is called “galvanizing.” Zinc mainly protects the steel by sacrificial action. Galvanized steel becomes PPGI after painting.
- PPGL (Pre-Painted Galvalume Steel) – Uses an aluminum-zinc (AZ) coating first. It is called “Galvalume.” Aluminum adds a strong oxide barrier, and zinc protects with sacrificial protection. It becomes PPGL after painting. PPGL is the ideal choice for harsh outdoor exposure.
Given below is a quick summary of PPGI vs. PPGL to understand easily.
Feature | PPGI | PPGL |
Name | Pre-Painted Galvanized Steel | Pre-Painted Galvalume Steel |
Base Metal | Galvanized steel | Galvalume steel |
Metallic Coating | Zinc (Z) | Aluminum-Zinc (AZ) |
Protection Mechanism | Sacrificial protection of zinc | Aluminum barrier + sacrificial protection of zinc |
Best Fit Environments | Inland, urban, standard outdoor | Coastal, humid, industrial, and other aggressive conditions |
Common Uses | Roofing, wall cladding, light industrial panels | Long-life roofing, cladding, harsh exposure projects |
After looking at the above details, you’ll get a clear idea regarding the pre-painted steel corrosion resistance. Simply, PPGI is designed for standard mild environments, and PPGL is specially developed for harsh conditions.
How to Choose the Right Base Coating Weight
Base coating weight (zinc or AluZinc) is the first and most important decision when selecting prepainted steel. It defines how much metallic layer protects the steel under the paint. Simply, if the coating thickness is higher, you’ll receive more corrosion protection.
Understanding G60, G90, Z275, and AZ70, AZ150 Coating Grades
You might have been confused with these different terms like G60, G90, Z275, AZ70, or AZ150. These all refer to coating weight, but they belong to different systems. These can be easily decoded as follows:
- G60 and G90 are zinc coating designations mainly used in ASTM standards (ounces per square foot).
- Z275 is also a zinc coating, expressed in metric units (275 g/m² total on both sides).
- AZ70 and AZ150 are aluminum-zinc (AluZinc) coating grades used for Galvalume steel.
One important point to note is that both G90 and Z275 are equivalent in corrosion performance. They are just expressed under different standards. Here’s a clear comparison of the above grades:
Coating Type | Pre-Painted Product | Explanation | Use Cases | Corrosion Resistance of Pre-Painted Steel |
G60 | PPGI | Light zinc coating | Indoor/mild outdoor | Moderate |
G90 | PPGI | Heavier zinc coating | General outdoor use | Better than G60 |
Z275 | PPGI | 275 g/m² zinc | Roofing, cladding | Equivalent to G90 |
AZ70 | PPGL | 70 g/m² Al-Zn | Mild environments | Moderate |
AZ150 | PPGL | 150 g/m² Al-Zn | Coastal / industrial | High |
When to Use Thicker Coatings for Durability
You must consider using thicker coatings when:
- Your product is expected to function in coastal or salty sites
- The project is in industrial conditions (with chemical fumes, pollution, dust)
- You want low maintenance and long service life
- The steel will face frequent wetting and drying (especially in roofing)
- Cut edges and fasteners are exposed and hard to maintain
If your goal is long life in harsh conditions, AZ150 is the safest base for PPGL steel.
Choosing the Right Paint System for PPGI and PPGL
After selecting the coating weight, the next key decision is the paint system. This is the outer paint layer that protects the steel from UV sunlight, rain, temperature variations, and other pollutants.
PE (Polyester) for Cost-Effective Paint Systems
PE is the most common and economical coating. It offers good appearance and standard outdoor protection. The PE paint system is best for:
- General roofing and wall panels
- Inland and urban projects
- Budget-focused construction
However, it also has limitations such as:
- Lower UV resistance compared to premium systems
- Faster fading under strong sunlight over time
SMP (Silicon Modified Polyester) for Better Durability
SMP improves regular polyester by adding silicon. This increases hardness and UV resistance. SMP paint systems are best for:
- Strong sunlight exposure
- Projects needing better color stability
- Commercial roofing with moderate durability needs
SMP is often considered a smart mid-range option between PE and PVDF.
PVDF (Polyvinylidene Fluoride) for Maximum Protection
PVDF is a premium coating known for excellent UV resistance and long-term color retention. It is the premium choice for harsh environments such as:
- Coastal buildings
- Premium architectural facades
- Long-life roofing and cladding
- Projects where fading is unacceptable
Although PVDF costs more, it reduces repainting risks and keeps buildings looking new for many years.
PE vs. SMP vs. PVDF: Quick Selection Guide
Paint System | Cost | UV Resistance | Use Cases |
PE | Low | Good | General projects |
SMP | Medium | Better | Hot climates, longer life |
PVDF | High | Excellent | Coastal, premium architecture |
Additional Considerations for Prepainted Steel
Now, you have a solid understanding of the right coating weight and paint system. However, there are a few more aspects to think about before finalizing your prepainted steel product. Many failures come from ignoring finishing, testing, and customization requirements.
Pre-Painted Steel Surface Finish Options
Surface finish decides the final appearance of the product, and sometimes performance. The common options you can go for are:
- Regular gloss or matte finishes
- Wrinkle finish for premium texture
- Embossed finishes for decorative panels
- Anti-fingerprint finishes (especially for appliances)
For roofing and wall panels, buyers often choose matte or low-gloss finishes to reduce glare and hide small handling marks.
Quality Control Tests for Pre-Painted Steel
This is a question you must ask suppliers before purchasing pre-painted steel. At least, the following two tests should be done to validate if the paint meets the required quality:
- T-bend test: Checks coating flexibility during bending and forming. Important for roll forming and sharp profiles.
- MEK rub test: Checks the cure quality and solvent resistance of the paint film. A poor result can mean an under-cured coating and an early failure risk.
There are several other quality tests that can evaluate the performance and the specifications of the paint:
- Coating thickness measurement (primer/topcoat/backcoat)
- Adhesion and cross-hatch testing
- Salt spray testing (for comparative QA)
- Color consistency testing
Our pre-painted coil coating processes at Zhaojian Steel are designed to follow ISO 9001 quality standards. Therefore, we can guarantee the performance levels and the quality of our pre-painted steel products.
Customization and Available Colors for Pre-Painted Steel
Color choice is not only an aesthetic consideration. Dark colors absorb more heat. Therefore, they may accelerate thermal aging. Lighter colors can help reduce heat load on roofing.
You must verify the details below with the supplier before finalizing the color:
- RAL color availability
- Gloss level options
- Protective film requirement
- Coil width, thickness, and tolerances
- Packaging for export and handling protection
How to Choose a Reliable Pre-Painted Steel Supplier
A good specification can still fail if the coating process of the supplier is wrong or poor. Your supplier choice matters as much as your coating choice.
Verifications Before Purchasing a Pre-Painted Steel Product
Before you purchase a pre-painted steel product, always check if the supplier has the following.
- Project references in similar environments
- Export experience and packaging quality
- Consistency of coating thickness and color batches
- Clarity of technical documentation (TDS, QC reports)
- Response speed for claims and after-sales support
- Certifications for quality standards
Also, you must ask whether they can support you with sample supply, consistent repeat orders, and stable lead times.
Certifications for Quality Standards: ISO, SGS, etc.
Certifications do not guarantee perfection, but they show process discipline. The most common quality standards almost all the reputed pre-painted steel suppliers own are:
- ISO quality management systems
- SGS inspection support (pre-shipment checks)
- Mill test certificates (MTC) with coating and mechanical data
- Compliance with relevant standards (ASTM / JIS / EN, depending on market)
If your project is high-risk (coastal/industrial), it’s better to request more proof, such as:
- Coating mass verifications
- Paint thickness breakdowns
- Bending and cure test results
- Salt spray or cyclic corrosion resistance test records, if available
Ready to make your choice? Get a free sample or request a quote for our high-quality PPGI and PPGL steel coils.
