The step nobody sees, the one that decides whether paint stays for ten years or fails in two. The chemistry and abrasion underneath every powder coat.
~70% of paint failures trace to surface prep, not the paint itself. Mill scale, oxide, and oils sit under the topcoat and delaminate it from below.
Zinc or iron phosphate forms a microscopic crystal layer that paint mechanically keys into. Without it, paint adheres only to whatever oils sit on the bare metal.
SA 2.5 is the standard to demand: near-white metal, no scale or oxide. Garnet or steel grit creates a 50–75 µm anchor profile that locks the primer to the substrate.
Bare blasted steel flash-rusts in 2 hours of Jeddah humidity. Prime within 60 minutes of blast or stage the workflow, the rust starts before the paint hits.
ISO 8501 cleanliness grades and conversion-coating weights ahead of paint or powder.
| Method | Grade · weight | Typical use |
|---|---|---|
| Solvent wipe (SSPC-SP1) | Pre-clean only | Oil, grease before blast |
| Brush-off blast (Sa 1) | Light cleaning | Re-touch, indoor low-spec |
| Commercial blast (Sa 2) | Standard | Indoor enclosures, generic |
| Near-white (Sa 2½) | Outdoor working min | All outdoor coastal Saudi |
| White metal (Sa 3) | Critical | Marine, splash zone |
| Iron phosphate | 3-5 g/m² | Indoor mild steel |
| Zinc phosphate | 2-3 g/m² (heavier) | Outdoor, coastal, salt-spray |
Most coating failures trace back to surface prep, not the coating chemistry.
Anything less leaves enough mill scale and rust to short-circuit the coating in 3-5 years. The extra cost of near-white over commercial blast is the cheapest paint-life insurance available.
Aim for 50-75 μm Ry5 for powder and standard liquids. Too shallow (under 25 μm), the coating slides off. Too deep (over 100 μm), the peaks pierce thin coatings and rust through the tops within a year.
Bare steel flash-rusts within hours in Saudi humidity. The blast → phosphate window is 4 hours max indoors, 1 hour coastal. Phosphate → paint window is 24 hours covered. Miss either and start over.
Aluminium oxide gives the sharp angular profile best for adhesion. Steel shot gives a rounded peening profile, weaker adhesion but compresses the surface. Glass beads are decorative only, no real prep value.
After blast, wipe with a white cotton rag. Visible dust → re-blow with oil-free air. Oily film → re-degrease. The lab solvent-extraction test (ISO 8502-3) is the spec; the rag test catches the field problems first.
Our 50,000 m² Dammam facility runs grit blast, phosphate spray and seven-tank pre-treatment in line. Every panel hits Sa 2½ minimum before it touches coat.
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