1.Why can the adhesion be better?
Ideally, sandblasting thoroughly removes all aged, chalking old coatings and contaminants, creating a perfect roughness (anchor pattern) on the galvanized layer or steel substrate surface. This provides a huge surface area and mechanical bonding points for the new coating, allowing adhesion (typically tested by the pull-out method) to easily reach over 10 MPa, far exceeding general standard requirements (e.g., ≥5 MPa).
In contrast, the original color-coated coil coating is applied over a relatively smooth galvanized passivation layer, relying mainly on chemical bonds and intermolecular forces. Sandblasting followed by recoating adds a strong mechanical anchoring force.
2.What are the assessment and preparation steps before sandblasting?
Assess the condition of the old coating: Inspect the degree of aging of the old coating (chalking, peeling, extent of rust). This determines the sandblasting grade.
Determine the cleanliness grade: For recoating, an ISO Sa 2.5 grade (near-white) is typically required. This means the surface should be free of visible oil, grease, dirt, scale, rust, paint, and foreign matter; any remaining traces should be only slight spots or streaks.
Select the appropriate abrasive:
Contraindicated: Quartz sand (generates silica dust, harmful to health), steel shot (too damaging to the galvanized layer).
Recommended:
Cast steel shot/steel grit (smaller particle size): High hardness, high efficiency, and controllable damage to the galvanized layer.
Non-metallic abrasives (e.g., alumina, garnet sand, plastic abrasive): Less damage to the substrate, less dust, especially suitable for applications where excessive thinning of the galvanized layer is a concern.
Mixed abrasives: To achieve ideal cleanliness and roughness.
3.How to control the core of the sandblasting process?
Roughness control: This is crucial for adhesion. The target roughness Rx (Ry) is typically between 30-70 micrometers. Too shallow (<20μm) results in poor anchoring; too deep (>80μm) excessively damages the galvanized layer and requires more paint to fill the peaks, potentially leading to premature coating failure.
Avoid over-blasting: Never completely remove the galvanized layer to expose the steel plate. Exposing the steel plate will create an electrochemical corrosion cell (the steel plate acts as the anode, accelerating corrosion). The surface after blasting should have a uniform silver-gray metallic luster (from the galvanized layer).
Cleanliness: The surface must be free of dust and abrasive residue after blasting.
4.How to select and apply coating systems?
Primer Selection: A primer compatible with the galvanized surface and possessing strong adhesion must be selected.
Preferred: Epoxy zinc-rich primer or epoxy primer. Epoxy resins exhibit excellent wet adhesion and corrosion resistance to metals.
Strictly Prohibited: Alkyd paints, ordinary oil-based paints, etc., as they will undergo a saponification reaction with zinc, leading to coating peeling.
Coating Compatibility: Follow the "primer-intermediate-topcoat" compatibility principle. The total dry film thickness of the recoating system should not be less than the original coating thickness, or should be designed according to the corrosive environment.
Application Environment: Temperature and humidity must meet the coating requirements to ensure proper curing of the coating.
5.What are the potential risks and common causes of failure?
Incomplete surface preparation: Old coatings, scale, or invisible contaminants remain.
Insufficient cleanliness: Dust or salt remains on the surface, forming a protective layer under the coating.
Over-blasting/exposed iron: This damages the continuous galvanized protective layer, creating a corrosion hazard; the coating will peel off at rust spots.
Incorrect primer selection: A paint incompatible with the galvanized layer was used.
Excessive interval between applications: A loose oxide film or white rust forms on the surface after blasting.