How to Handle Rust Stain Seepage on Exterior Wall GRC: Corrosion Mechanism Diagnosis and Systematic Repair Solutions

The appearance of rust seepage on the surface of exterior wall GRC (commonly known as "rust bleed-through") is a serious quality defect signal. It not only affects aesthetics but also indicates that internal metal connectors or reinforcement may be corroding, threatening structural safety. Based on extensive engineering repair cases, Qinglong Group attributes the root causes of rust bleed-through to three major categories: corrosion of embedded iron parts, corrosion of steel mesh, or external contamination. Treatment must follow a systematic process of "diagnosis – treatment – prevention," and is by no means a simple surface cover-up. For example, after sporadic rust spots appeared on a coastal project, our professional testing revealed that they were caused by damage to the galvanized coating of local embedded parts; through a combined repair solution, we successfully eliminated the problem and prevented recurrence.

Accurate diagnosis is the first step toward successful repair. First, non-destructive detection is performed, using a rebar scanner to locate internal metal parts, combined with the tapping method to identify hollow areas. Then local invasive sampling is conducted to observe the color and form of the rust: brownish-red powder usually indicates corrosion of iron parts, while reddish-brown rust accompanied by cracks along the reinforcement may indicate rebar corrosion expansion. We once encountered "pseudo" contamination in which rust-colored water flowed onto the GRC surface due to improper drainage from exterior aluminum panels; this was distinguished through composition testing. Based on the diagnostic results, a targeted plan is formulated: if it is localized embedded part corrosion, slotting and local treatment is carried out; if it is large-area rebar corrosion, systematic replacement may be required.

For localized corrosion of embedded parts, our standard repair process consists of six steps: first, precise slotting along a boundary extended 50mm beyond the rust spread until the corrosion source is exposed; second, thorough removal of all rust using sandblasting or an angle grinder until a metallic sheen is revealed; the third step is applying a polymer rust converter to transform residual iron ions into a stable substance; the fourth step is anti-corrosion treatment with epoxy zinc-rich primer, with a dry film thickness of no less than 80μm; next, layered restoration using polymer repair mortar compatible with the substrate, with each layer no thicker than 10mm, pressing in alkali-resistant mesh for reinforcement; finally, surface texture and color repair to ensure seamless integration with the surroundings. Throughout the process, ambient humidity must remain below 80% to ensure proper curing of the repair materials.

Long-term prevention after completion of the repair is crucial. We recommend replacing all exposed metal parts, such as installation anchors and bolts, with 316 stainless steel or treating them with hot-dip galvanizing plus a sealing coating. At the same time, inspect and repair any damaged areas of the GRC protective layer, ensuring its thickness is no less than 15mm. Apply a penetrating silane protective agent to the repaired areas and their surroundings to form a long-lasting hydrophobic layer. Establish a regular inspection system, especially checks after rain, to ensure early detection and early treatment. Through this complete technical system spanning eradication to prevention, the stubborn problem of rust bleed-through on GRC exterior walls can be effectively resolved, restoring and extending their service life.