How to Deal with Rust Stain Seepage on Exterior Wall GRC? Rust Treatment and Qinglong Group's Source Prevention and Control System

Rust stains appearing on the surface of exterior wall GRC (commonly known as 'rust reappearance') are a clear signal of corrosion in internal metal embedments or rebar, requiring immediate systematic treatment of 'diagnosis, treatment, eradication, and prevention'; otherwise, rust expansion stress will cause the GRC cover layer to crack and spall, endangering safety. Through 28 years of maintenance practice, Qinglong Group has developed a standard treatment process: first, precisely locate internal corrosion points using infrared thermal imaging and a rebar detector; then chisel open the cover layer (over an area at least twice the size of the rusted surface), thoroughly remove the corroded metal, and apply a migrating rust inhibitor; next, restore with polymer repair mortar and apply an anti-carbonation coating to the surface; finally, investigate and eliminate contributing causes such as water seepage and chloride ion intrusion. In the repair of a coastal project, we successfully treated 37 rust stains using this method and applied preventive spraying to all non-corroded embedments throughout the building; a review three years later showed zero recurrence. Treating rust is like treating an illness—both symptoms and root causes must be addressed.

Accurate Diagnosis Is the First Step to Effective Treatment

The pattern of rust stains provides clues: dot-shaped rust marks often indicate localized corrosion of embedments, while strip-shaped rust marks are usually corrosion along rebar. Professional equipment is needed to determine the depth and extent of corrosion. We use an endoscope to examine internal conditions, assess the impact of corrosion on load-bearing capacity, and decide between localized repair and component replacement. For critical load-bearing components, a structural safety assessment is mandatory.

Complete Eradication and Scientific Repair Are the Core

The corroded metal and the surrounding contaminated, loose GRC must be completely chiseled away until a solid substrate is exposed. For the cleaned metal surface, rust is removed with power tools to St2 grade, and an epoxy zinc-rich primer or specialized rust inhibitor is applied immediately. The repair material should be a polymer mortar matched to the elastic modulus and shrinkage rate of the GRC matrix, applied in layers with proper curing to ensure a strong bond between new and old interfaces.

Surface Sealing and Protection Provide Long-Term Assurance

After the repaired area has dried, a penetrating anti-carbonation coating or a finish coating harmonized with the surroundings must be applied to block water, oxygen, and chloride ions. More importantly, the root causes of corrosion must be investigated and repaired, such as cracked joints, failed waterproofing, or problems that existed from the design stage, such as insufficient cover thickness or the use of sea sand.

Source Control: Prevention Over Treatment

Qinglong Group strictly implements anti-corrosion standards at the production stage: using galvanized or stainless steel embedments; ensuring rebar cover thickness of ≥20mm (outdoor) with controlled deviation; strictly prohibiting chloride-containing admixtures or sea sand; and applying specialized anti-rust treatment to weld points. Eliminating hazards at the source is far more economical and reliable than repairs after the fact.

Rust bleeding to the surface is a 'health alarm' for GRC exterior walls and must never simply be covered up with surface painting. Qinglong Group's professional treatment solution aims to eradicate the root cause, restore performance, and establish a long-term health monitoring mechanism.