1.Why is the lightfastness compromised after touch-up?
Incomplete coating system:
Original coating: substrate → chemical conversion layer (such as phosphate layer) → primer (usually epoxy, with strong adhesion and anti-rust) → topcoat (polyester, silicon-modified polyester, PVDF, etc., providing color and weather resistance). This is a complete, layer-by-layer coordinated protection system.
On-site touch-up: usually only 1-2 layers of topcoat or universal touch-up paint can be applied directly after cleaning the scratches. The key chemical conversion layer and dedicated primer are missing, resulting in a weak adhesion foundation.
Construction conditions vary widely:
Original factory process: pre-treatment in a clean workshop, then evenly painted by roller coating, and quickly baked and solidified at high temperature (PMT about 200-240°C) to form a dense, fully cross-linked paint film.
On-site touch-up painting: manual brushing or spraying, normal temperature or natural curing. The leveling, thickness uniformity and curing degree of the paint film are far inferior to those of the original manufacturer. A paint film that is insufficiently cured will inevitably have poor weather resistance.
2.What is the impact of material match issues?
It is difficult to find refinish paint that is exactly the same chemical composition and weather resistance level as the raw material. Even if the colors are close, the resin system (such as using ordinary alkyd paint to touch up the PVDF topcoat) may not be compatible, resulting in poor bonding and accelerated aging.
The content and performance of UV absorbers, stabilizers and other additives cannot be compared with original products.
3.How is the surface treated?
Cleaning: Use solvent (such as special thinner) to completely remove oil and dust in and around the scratched area.
Sanding: Use fine sandpaper (such as 400# or above) to polish the scratched area into a smooth slope shape (feathered edge), and expand the sanding area to increase adhesion. Thoroughly remove sanding dust.
Anti-rust: If iron is exposed, be sure to apply special anti-rust paint or epoxy zinc-rich primer to completely cover the exposed metal area.
4.How to make material selection?
It is best to contact the manufacturer or supplier of color-coated coils to purchase special repair paint supplied by the original manufacturer.
Secondary option: Use well-known brands of outdoor acrylic polyurethane paint and fluorocarbon paint repair agents, which have relatively good weather resistance. Always confirm compatibility with the substrate and original coating.
5.Any suggestions for patching?
For non-critical parts, non-severely corrosive environments, and small-area scratches: repairing according to the above-mentioned rigorous process can be used as a maintenance method to extend the overall service life, but you need to accept the color difference and the fact that it may be partially re-patched in the future.
For critical parts, highly corrosive environments (such as chemical industry, coastal areas, high UV areas) or large-scale damage: repair is not a reliable long-term solution. The safest solution is to replace the entire color-coated plate to ensure the sealing and durability of the entire equipment.
Important reminder: If this color-coated coil is used in the chemical waste gas treatment environment mentioned in your previous question, then any repairs are only temporary emergency measures, because chemical corrosion will quickly penetrate from the repaired area, and replacement with more corrosion-resistant materials (such as FRP, stainless steel, etc.) should be planned as soon as possible.