Pre-painted Galvalume® Sheet:
A Guide to Best Practices
Steel Roofing The manufacture of prepainted GALVALUME sheet panels starts with the GALVALUME steel substrate and its exceptionally corrosion-resistant aluminum-zinc alloy coating. Next, this high-performance coated steel passes through modern high-speed continuous coil coating lines that carefully apply and oven-cure the paint systems. The painted product is made to rigid specifications and quality-tested to meet critical laboratory performance requirements established by ASTM and paint manufacturers in cooperation with the National Coil Coaters Association (NCCA). After painting, the coils are ready for forming into versatile, eye-catching roofing and siding panels.
Prepainted GALVALUME sheet is supplied with a factory-applied, baked-on finish. Processing on the paint line includes cleaning, pretreating, priming and top coating with a uniform paint finish, all in one continuous process.
A schematic diagram of the prepainted GALVALUME sheet system is shown in Figure 1 (below). GALVALUME sheet, with its highly corrosion-resistant aluminum-zinc coating, serves as the foundation for the paint system. A chemical pretreatment is applied to promote adhesion between the paint and metal coating surface. A high-performance primer is applied to the pretreated surface to enhance corrosion resistance, particularly at cut edges, scratches and bends. The system is completed by applying a colorful, durable paint selected for the performance requirements of the application. Typical topcoats include acrylics, polyesters, siliconized polyesters, fluoropolymers and plastisols.
Figure 1 — Prepainted GALVALUME Sheet System
Durable, beautiful finishes acrylics, polyesters, siliconized polyesters, fluoropolymers, plastisols in almost any color you want (Layer 1).
High-performance primers are key to long-term corrosion resistance and paint adhesion (Layer 2).
Pretreatments provide excellent corrosion resistance and adhesion (Layer 3).
An aluminum-zinc alloy coating about 55% aluminum, 45% zinc. Quite a force in fighting corrosion (Layer 4).
Uniform steel sheet properties are a result of modern steelmaking technology (Layer 5).
Backer coats to enhance corrosion resistance and minimize abrasion damage (Layer 6).
It is recommended that the unexposed or back side be pretreated, primed and coated with a 'backer' coat. The backer coat is applied to enhance performance and appearance, and to minimize abrasion during shipping and handling. The back side may be coated with a full paint system similar to the exposed side. Refer to paint manufacturers for specific recommendations.
Table 1, prepared by NCCA, provides a guideline for selection of generic paint systems for particular applications. Coating systems should be specified with the assistance of technical service representatives from the GALVALUME sheet producers, coil coaters and paint manufacturers.
Rollforming is most commonly used to form prepainted GALVALUME sheet into roofing and siding panels. Rollformers consist of a series of shaped rolls mounted on stands that progressively form the sheet fed from a coil into panel profiles (Figure 2).
Prepainted GALVALUME sheet can be readily formed into an almost limitless variety of shapes and profiles, depending on the strength and ductility of the GALVALUME sheet, the characteristics of the paint system, and the type of rollforming equipment and its maintenance (Figure 3).
Figure 3 — Some Rollformed GALVALUME Profiles
To enhance the field performance of prepainted GALVALUME sheet, especially at formed areas, the good practice guidelines in Table 2 (below) should be followed.
Table 2 — Good Practice Guidelines for Rollforming Prepainted GALVALUME Sheet
- Use panel designs with large bend radii. It is possible to design architecturally desirable, deep profiles, and at the same time, use large bend radii.
- Design the rollformer so that profiling occurs from the center of the sheet out towards the edges.
- Design the rollformer to accommodate the strength of the steel grade. For example, a higher strength steel may require more rollformer stands to successfully produce a given profile.
- Use lower strength steels to allow greater design latitude when making more complex profiles.
- Maintain the rollforming equipment to achieve proper vertical and horizontal tooling alignment, as well as adequate clearance between the mating top and bottom spindle rolls to avoid abrading the coating. Clean the rolls to avoid damage to the paint finish.
- Incorporate the appropriate number of rollforming stages to ensure smooth transitioning of bends.
- Use flexible paint/primer systems to maximize performance at bends.
Other Types of Forming
Rollforming is by far the most common type of forming for prepainted GALVALUME sheet. However, for applications where prepainted GALVALUME sheet is used for wall and/or roof panels, there will very likely be a need to press-brake form trim parts and other accessories. Again, it is important to use the maximum permissible bend radii to enhance the performance at bends.
Equally important for press-brake forming is the need to protect the painted surface from scratches and abrasion. This can be accomplished by using polyurethane or a similar hard plastic material for the dies. Proper tooling clearances should be maintained during this operation.
Roll embossing is another type of forming that can be applied to prepainted GALVALUME sheet. Care must be exercised to assure that the paint and metallic GALVALUME coating are not damaged during embossing. The depth and sharpness of the embossment should be sufficiently gentle to assure that the durability of prepainted GALVALUME sheet is not compromised.
Other types of forming, including stamping, notching and piercing can be applied to prepainted GALVALUME sheet. Many of the factors discussed previously, such as die clearance and good paint system flexibility, are equally important considerations for these types of forming. For example, stamping is used to form prepainted GALVALUME facsimile roof tile panels, mainly used for residential roofing.