1. What is "chalking" in galvanized pre-coated steel coils?
Chalking in galvanized pre-coated steel coils is a phenomenon caused by the aging of the coating under outdoor conditions. You can think of it as the paint "peeling off" in the sun.
Specifically, the resin in the coating degrades under the influence of ultraviolet light, temperature, and humidity, losing its ability to bind pigment particles. When these unbound pigment powders float on the surface, chalking occurs. If you gently rub the coating surface with your hand and your fingers are covered with noticeable white or light-colored powder, it means chalking has occurred. It's important to note that chalking is a normal part of the coating aging process, and almost all pre-coated steel sheets cannot be completely avoided after long-term outdoor use.
2. What are the core reasons for powdering in galvanized color-coated coils?
A: Powdering is mainly caused by two factors: "inherent quality" and "environmental factors."
"Inherent" factors: These are the basic quality of the material at the time of manufacture.
Poor coating quality: This is the most important internal factor. For example, using ordinary polyester (PE) coatings that are not resistant to ultraviolet light, or coatings that are too thin (e.g., topcoat thickness less than 20μm). Improper curing processes (insufficient baking temperature or time) can also lead to insufficient resin cross-linking, making the coating more prone to decomposition.
Poor substrate treatment: Incomplete removal of oil, rust, etc., from the substrate surface will greatly weaken the coating adhesion, making it easier to peel off.
"Environmental" factors: These are the continuous tests posed by the external environment to the material.
Ultraviolet radiation (UV) is the culprit: Ultraviolet radiation from sunlight is the number one factor leading to coating degradation, especially in areas with high UV radiation, where the powdering process is significantly accelerated.
3. How to assess the severity of chalking? (National standards apply)
A: The first step in dealing with chalking is to accurately assess its severity. This can be determined according to the chalking grades specified in GB/T 1766-2008. They are usually divided into grades 0-4, with higher grades indicating more severe chalking.
Chalking Grade | Phenomenon Description | Treatment Recommendations
Grade 0 (No chalking) | The coating surface is intact, with no signs of chalking. Normal use and regular cleaning are sufficient.
Grade 1 (Slight) | Wiping with hand or a dark cloth leaves a small amount of powder on the fingertips or cloth. | Minor repair, such as sanding or polishing.
Grade 2 (Moderate) | Obvious chalking on the coating surface, but not yet extensive peeling. | Moderate repair, such as sanding and recoating.
Grade 3 (Severe) | Severe chalking, with a large amount of powdery material peeling off the coating surface. | Thorough treatment, complete recoating required.
4. What are the specific treatment plans for different degrees of chalking?
A: Based on the assessment results of the chalking level, a graded treatment plan can be adopted, ranging from simple cleaning to complete recoating.
Level 1 (Slight Chalking): Cleaning and Polishing
Cleaning: Use a neutral detergent and a soft cloth/brush to thoroughly remove dust and contaminants from the surface.
Polishing: After drying, wet sand with fine sandpaper (e.g., 1000 grit or higher) or polishing wax to carefully remove the chalking layer from the surface.
Protection: After polishing, a layer of special protective wax or sealant can be applied to delay re-chalking.
Levels 2-3 (Moderate to Severe Chalking): Sanding and Recoating
Sanding: Thoroughly sand the chalking layer with medium-grit sandpaper (e.g., 180-240 grit) until a solid, clean old coating or substrate is exposed. This step is crucial to ensuring the adhesion of the new coating.
Cleaning: After sanding, thoroughly remove dust with compressed air or clean water.
Primer: Apply a layer of primer compatible with the board and topcoat to enhance corrosion resistance and adhesion.
5. Are there any long-term solutions to prevent or delay chalking?
A: Yes, prevention at the source is always the most economical and efficient method.
Start with selection: Choose high-performance coatings
Prioritize high-weather-resistant coatings: In outdoor areas, areas with strong ultraviolet radiation, or industrial pollution areas, prioritize high-performance coatings such as fluorocarbon (PVDF) or high-weather-resistant polyester (HDP). Their weather resistance far exceeds that of ordinary polyester (PE), effectively delaying chalking.
Ensure sufficient coating thickness: Clearly specify the topcoat thickness in the contract; for outdoor boards, a minimum of 20μm is recommended. Insufficiently thin coatings are one of the direct causes of early chalking.
Start with construction: Pay attention to details and protection
Protect cuts: After cutting or drilling, the edges of the board will expose metal. These must be immediately sealed with anti-rust paint or sealant to prevent corrosion from spreading inwards from the edges.