Common Issues In Color-Coated Steel Coils In 2025

Common Issues in Color-Coated Steel Coils in 2025

Color-coated steel coils are widely used across industries such as construction, home appliances, transportation, and more due to their attractive finish, durability, and corrosion resistance. However, specific quality issues may arise during production and processing, affecting performance and appearance. Below is a comprehensive guide to the most frequent problems seen in pre-painted galvanized steel (PPGI) or pre-painted galvalume steel (PPGL) coils, including their potential causes and suggestions for mitigation.

You've probably seen color-coated steel coils everywhere-they're the ​​unsung heroes​​ behind modern buildings, shiny kitchen appliances, and even those eye-catching food trucks rolling down your street. With their ​​flashy colors, ​​rust-proof superpowers, and ​​tough-as-nails durability, it's no wonder industries from construction to car manufacturing swear by them.

But here's the kicker: even superhero materials have weaknesses. If you've ever dealt with ​​peeling paint, ​​weird color streaks, or ​​bubbles​​ on pre-painted steel (aka PPGI or PPGL coils), you know how frustrating "steel fails" can be. Let's break down the ​​top 10 issues​​ that mess with color-coated steel-and exactly how to fix them.

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1. Poor Adhesion

Coating adhesion failure occurs when the paint layer does not bond properly to the steel surface, often revealed during tape tests or cross-hatch tests. This can result from surface contamination, improper pre-treatment, or incorrect baking temperature.

2. Missed Coating (Uncoated Areas)

Missed coating spots appear when sections of the coil surface are left without paint. This is typically caused by inconsistent paint application, clogged nozzles, or poor control of the coating head.

3. Color Difference

Inconsistent color or tone across the coil surface may result from poor paint mixing, different baking conditions, or raw material inconsistencies. Color difference is especially critical for visible exterior applications.

4. Edge Wave

Wavy edges on the coil strip can appear during slitting or cutting due to tension imbalance or improper coil handling. It impacts both aesthetics and usability.

5. Shrinkage Cavity (Cratering)

Small pits or craters seen on the surface after curing may be caused by oil or dust contamination, paint gelling issues, or poor surface cleaning.

6. Low Gloss

Gloss reduction can result from paint formulation, insufficient curing, or improper line speed. It affects the surface reflectivity and overall visual appeal.

7. Pinholes

Tiny holes in the coating layer often stem from solvent boiling, poor degassing, or contamination. These can reduce corrosion resistance.

8. Surface Transverse Stripes

Stripes or uneven surface patterns may occur from uneven coating thickness, wrinkling during baking, or paint flow issues.

9. Back Staining

When the back of the coil is stained during stacking or rewinding, it often points to excessive coating thickness or improper cooling during winding.

10. Low Pencil Hardness

Pencil hardness is a measure of surface hardness. A low rating can indicate poor curing or an incorrect coating formulation, leading to scratch vulnerability.

11. T-Bend Failure

Poor T-bend performance shows that the coating cracks or peels during forming. It may be due to insufficient flexibility in the paint or over-curing.

12. Bubbles (Blistering)

Air or solvent trapped under the coating can form bubbles when heated. This often happens due to surface oil, insufficient drying, or moisture issues.

13. Edge Bubbling

Edge bubbling during curing is typically caused by excessive paint at the edges or poor coil edge quality.

14. Roll Marks

Roller defects or debris on equipment can leave visible linear marks on the coil surface. Regular maintenance can help avoid this.

15. Scratches

Scratches can be introduced during handling, transportation, or improper stacking. They affect aesthetics and may lead to localized rusting.

16. Poor Scratch Resistance

If the topcoat lacks scratch resistance, it may wear off easily. This is usually due to weak resin systems or incorrect curing temperatures.

17. Gloss Spots

Patchy gloss levels across the surface are signs of uneven paint flow or variations in coating thickness during application.

18. Insufficient MEK Resistance

MEK (Methyl Ethyl Ketone) resistance testing reflects coating hardness and curing. Poor MEK resistance indicates under-cured paint and weak durability.

19. White Rust

White rust, usually a zinc corrosion product, forms on galvanized layers when coils are exposed to moisture. Proper packaging and storage can prevent it.

Understanding these quality issues helps manufacturers and buyers make sure that color-coated steel coils always perform well and last long. At Promisteel, we stick to tough QC rules and keep improving our processes. This way, we can send out trustworthy, top-notch PPGI and PPGL products to customers all over the world.