Technical Article
Surface Preparation of Rusted Metal Roof Sheets. Why ST2 Matters.
Technical Article
Surface Preparation of Rusted Metal Roof Sheets. Why ST2 Matters.
When refurbishing a metal roof, surface preparation is the most important factor in achieving a long-lasting coating system. No matter how good the coating is, its performance depends on the condition of the substrate beneath it.
What Is ST2 Surface Preparation?
ST2 is a surface preparation standard defined in ISO 8501-1 and refers to thorough hand and power tool cleaning. This involves removing all loose rust, loose paint, loose mill scale, and other contaminants using tools such as rotary wire wheels, scrapers, sanders, or mechanical preparation equipment
The objective is to create a clean, sound surface that provides a suitable base for coating application.
Why Prepare Rust to ST2 Standard?
Rust is an unstable and porous material. If coatings are applied over loose or poorly adhered rust, corrosion can continue beneath the coating film.
Preparing roof sheets to an ST2 standard:
• Removes loose and unstable corrosion.
• Improves coating adhesion of a new coating system.
• Reduces the risk of underfilm corrosion.
• Is essential for warranty validation and extends the life of the coating system.
• Improves the overall finish and appearance.
Proper preparation provides the foundation for a successful roof refurbishment.
What Happens If Rust Is Not Properly Removed?
Applying a new coating directly over rust often leads to premature coating failure.
Common issues include:
• Rapid re-rusting and continued corrosion beneath the new coating.
• Blistering and peeling coatings.
• Loss of adhesion.
• Associated repair costs.
In many cases, rust will quickly reappear (within 12 months) because the corrosion process was never properly stopped before coating application.
The Importance of a High-Performance Rust-Inhibiting Primer
Following ST2 preparation, exposed steel should be immediately protected with a high-performance rust-inhibiting primer.
These primers are specifically designed to:
• Prevent moisture and oxygen reaching the steel.
• Provide long-term corrosion protection.
• Seal the prepared steel surface.
• Improve adhesion between the substrate and topcoat.
A quality primer forms the foundation of the coating system and plays a critical role in maximising the lifespan of the refurbishment.
CONCLUSION
Preparing rusted metal roof sheets to an ST2 standard is essential for achieving a durable and effective refurbishment coating system. Without proper preparation, corrosion can continue beneath the coating, leading to early failure and costly remedial work.
Combined with a high-performance rust-inhibiting primer, ST2 surface preparation helps ensure maximum coating adhesion, corrosion resistance, and long-term roof protection.
