Prepainted Metal Sheets

Prepainted metal sheets are widely used in the construction industry. They can be seen on roofs, wall panels, building enclosures, wall coverings, etc.

Different buildings have different requirements for prepainted metal sheets. For example, civil buildings pay the most attention to safety and service life, while public buildings pay more attention to aesthetics, service life, bearing capacity, etc. Let’s learn how to choose prepainted metal sheets for construction.

How to Choose Prepainted Metal Sheets for Construction

The following factors need to be considered when selecting pre-painted metal sheets for construction uses:

1. Substrate type and grades, including the yield strength, tensile strength, and elongation of the base metal;

2. Coating, such as the coating thickness, adhesion, and coating types (GI, GL, ZAM, etc.);

3. Paint, including the color difference, gloss, T-bending test, impact test, hardness, chalk resistance, heat and humidity resistance, etc.

4. Whether the surface has visible surface defects, matte or glossy finish, embossed or plain surface, etc.

5. Design, including the tolerance, roughness of the surface, etc.

Here is the structure of the PPGI sheet, which may help you understand it well.

1. Steel Grades – Base Metal

Steel strength matters in the bearing capacity of buildings, the span, the design of the purlin, etc. There are corresponding standards at home and abroad for color-coated sheets and galvanized sheets for building uses, such as GB/T 12754-2006, ASTM653, JISG3302, EN10215, etc.

The steel sheets are divided into CQ (commercial uses), DQ (general stamping), HSS (high-strength structural steel), and FH (full-hard steel).

CQ is the most commonly used steel grade. For example, the yield strength of DC51 is between 260-320MPa, and its tensile strength is between 350-400MPa. Compared with structural steel, its yield-to-strength ratio is slightly higher.

DQ is mainly used in doors, windows, pipes, and other industries.

HSS: Steel with a yield strength of 280MPa and 345MPa is mostly used, namely S280GD and TS350GD. Compared with CQ steel, HSS steel has some advantages:
1) Due to the relatively low yield strength, HSS has good fire resistance and shock resistance;
2) For thicker steel plates, it can manufacture large-span arched plates;
3) For exterior wall and roof panels, it can be appropriately thinner than CQ material, thereby reducing material consumption.

FH: It is a type of steel that improves yield strength by controlling the annealing temperature. According to the international standard, the yield strength is ≥550MPa, and the tensile strength is ≥570MPa. This type of material has high strength and low elongation.

2. Coating & Paint

The coating and paint aim to improve the durability and corrosion resistance of the building. We should use different coatings, paints, and thicknesses for different uses and design life.

Coating

The commonly used base metals for buildings are hot-dip galvanized steel (pure zinc coating) and galvalume steel (Al-Zn coating). In acidic environments, GL steel is a better choice. For humid and alkaline environments, galvanized steel is better. The coating thickness of HDG for roof panels and exterior wall panels is ≥180g/m² (about 26 μm) and that of GL is ≥100 g/m² (about 27 μm). For heavy industrial areas or seaside, the zinc layer thickness is ≥275g/m² (about 38 μm), and the Al-Zn coating thickness is ≥150 g/m² (about 40 μm).

Paint

Paint can meet the user’s demand for color and durability. When ordering, it is necessary to specify the type of paint, color and gloss, and coating thickness.

The commonly used top paint are polyester (PE), HDP (high durability polyester), silicon modified polyester (SMP), and PVDF (polyvinylidene fluoride). The primer and back paints are epoxy (EP), polyester (PE), polyurethane (PU), and so on. Below are their features for your reference.

Paint Types	Hardness	Bending performance	Corrosion resistance	Weather resistance	Cost
PE	Excellent	Good	Good	Great	Low
SMP	Good	Medium	Great	Excellent	Medium
HDP	Good	Excellent	Great	Excellent	Medium
PVDF	Medium	Excellent	Excellent	Excellent	High

3. How to choose color

When choosing the color of prepainted metal sheets, we usually choose color matching according to the surrounding environment and personal preferences.

However, different colors reflect light differently, because different colors have different glossiness, and glossiness can measure the ability of the coating surface to reflect light.

Reflection ability of different colors

Color	Reflectivity	Reflective effect	Color	Reflectivity	Reflective effect
White	84%	Best	Light green	54.10%	Better
Milky white	70.40%	Better	Light blue	45.50%	Medium
Light red	69.40%	Better	Brown	23.60%	Poor
Beige	64.30%	Better	Black	2.90%	Worst

In addition, different colors also affect the performance of prepainted metal sheets. The following table shows the results of atmospheric exposure tests of different colors in the same location (coastal area) and the same supplier for 8 years:

Comparison table of atmospheric exposure test results of different colors of prepainted metal sheets for 8 years

Color	Original gloss (%)	Current gloss (%)	Color difference (AE)	Original coating thickness (um)	Current coating thickness (um)	Powdering level
White	35	3	1.49	24	20	2
Red	45	3	1.34	24	20	2
Light yellow	26	2	1.26	22	17	2
Green	41	2	3.43	24	17	2
Blue	41	2	4.95	24	17	2

It can be seen from the table that color has a significant effect on the performance of prepainted metal sheets.

The coating color has a significant effect on the aging resistance of prepainted metal sheets. The color difference and coating thickness variation of color-coated metal sheets with low heat absorption coefficient and high reflectivity (white, red, light yellow) are significantly smaller than those of green and blue with high heat absorption coefficient and low reflectivity.

Therefore, when choosing colors, you should choose colors with low heat absorption coefficient and high reflectivity. Generally speaking, light colors have high heat reflectivity, strong heat reflection ability, and good color retention.

Notice: Try to use the pre-paint metal sheet  from the same supplier in the same building because the powder speed of different paints is different. Even if the color seems to be similar when the building is completed, it may look different after a period of time, which will affect the appearance.

Microscopically, the paint is porous. So water and corrosive media will invade through the weak part of the paint, causing sub-film corrosion, and then the paint will peel off. In addition, even with the same paint thickness, paint twice is denser than a single painting. So the recommended top paint thickness for construction use is more than 20μm. And PVDF-coated steel requires thicker painting.

Prepainted metal sheets price

Wanzhi Steel produces prepainted metal sheets including:

• Prepainted galvanized steel
• Prepainted galvalume steel
• Prepainted aluminum zinc magnesium steel
• Prepainted aluminum coil

The prices of the above products vary. The prices are affected by factors such as material, specification, color, paint, and market demand. Therefore, we cannot provide specific price information. If you need specific price information, you can consult us.

The approximate price comparison of these four products is: prepainted galvanized steel price < prepainted galvanized steel price < prepainted galvanized magnesium steel price < prepainted aluminum coil price.

Conclusion

Prepainted metal sheets are widely used in civil buildings, shopping malls, industrial plants, hotels, resorts, farmhouses, office buildings and other public buildings. When purchasing prepainted metal sheets for construction, you need to consider their use environment, purpose, expected service life, and choose the right substrate, coating, paint, color, etc.

Wanzhi Steel has rich experience in the production of prepainted steel sheets. We have helped more than 260 customers complete projects. In addition to ordinary prepainted steel sheets, we also sell antibacterial prepainted paint, nano-coated sheets and anti-static coated sheets. We are confident that we can provide you with the right solution. If you have any questions, please feel free to contact us.