One of the most common errors in steel structure construction — and the most costly in terms of repairs — is improper technical surface treatment of steel before painting.
Many investors see a shiny, beautiful paint finish upon completion, not realizing that if the underlying steel surface was not cleaned to standard, the paint will peel and rust will begin to spread after only 12–24 months — regardless of how expensive the paint is.
This is essential knowledge that every investor and project supervisor for steel structures must master.
Why must the steel surface be cleaned before painting?
Hot-rolled steel from the factory always has a blue-gray mill scale layer firmly attached to the surface. This layer:
- Prevents paint from adhering to the actual steel surface
- Has a different electrochemical potential than the underlying steel → creates an electrochemical cell → accelerates corrosion
- Is brittle and peels easily when the steel deforms → pulls the paint layer off with it
In addition to mill scale, the steel surface also contains rust (from storage), oil, grease, dirt, and welding slag — all of which are enemies of the protective paint layer.
What is Shot Blasting? Why not sanding or sandblasting?
Shot blasting is the process of propelling steel shot or steel grit at high velocity (60–80 m/s) onto the steel surface using a centrifugal wheel, creating a powerful mechanical impact to:
- Completely remove mill scale, rust, and dirt
- Create an anchor profile to help the paint adhere more firmly
- Achieve uniform cleanliness across the entire surface area of the component
Comparison of cleaning methods:
| Method | Cleanliness achieved | Cost | Speed | Common for large steel structures |
|---|---|---|---|---|
| Manual sanding | St 2 (low) | Very low | Very slow | None |
| Grinding rust removal | St 3 | Low | Slow | None |
| Sandblasting | Sa 2 – Sa 3 | Medium | Medium | Limited (pollution) |
| Shot blasting | Sa 2.5 – Sa 3 | Medium | Fastest | Industry standard |
When you see a technical spec requiring “cleaning to Sa 2.5”, this is the cleanliness level according to ISO 8501-1 (SIS 055900) standards:
- Sa 1: Light cleaning — removes loose materials. Not sufficient for protective coating.
- Sa 2: Thorough cleaning — removes most mill scale and rust. Some faint traces remain.
- Sa 2.5: Very thorough cleaning — almost all mill scale, rust, and old paint are removed. The surface has a uniform metallic gray color. This is the minimum standard for industrial epoxy coating.
- Sa 3: Cleaning to white metal — 100% removal. Required for extremely corrosive environments.
Surface roughness – the second equally important factor
Shot blasting not only cleans but also creates surface roughness — a micro-structure of peaks and valleys on the steel — helping the paint “anchor” into the surface rather than just sticking to it.
Commonly required roughness: Rz = 40–70 micrometers (measured by profilometer or compared with Testex Press-O-Film standard samples).
If the roughness is too low (surface too flat): the paint will not adhere well. If the roughness is too high (peaks too sharp): the peaks may penetrate the thin paint layer, creating localized corrosion points.
Steel structure coating system
Protective coating for steel structures is not a single layer — it is a multi-layer system where each layer has its own function:
Layer 1: Anti-corrosive primer
Function: Adheres to the steel surface and prevents moisture penetration.
Common types:
- Zinc-rich epoxy primer: Zinc acts as a sacrificial anode to protect the steel. Standard for FDI projects.
- Alkyd primer: Lower cost, used for less corrosive environments.
Dry film thickness: 60–80 microns (measured by dry film thickness gauge)
Layer 2: Intermediate/Tie Coat
Function: Increases the thickness of the coating system and improves moisture resistance.
Common type: Epoxy mastic — good adhesion, moisture resistant, chemical resistant.
Dry film thickness: 75–100 microns
Layer 3: Topcoat
Function: UV protection, atmospheric corrosion resistance, provides color and aesthetic finish.
Common types:
- Polyurethane topcoat: Glossy, UV resistant, abrasion resistant. Used for outdoor structures.
- Acrylic polyurethane: Excellent color retention and UV resistance.
Dry film thickness: 50–75 microns
Total dry film thickness for FDI factory paint systems: 180–255 microns (DFT — Dry Film Thickness)
Common mistakes to avoid
Mistake 1: Painting when the surface is damp – Relative humidity >85% or steel surface temperature is less than 3°C above the dew point → paint will not cure properly and is prone to blistering.
Mistake 2: Excessive time between shot blasting and priming. Steel must be primed within 4–6 hours after shot blasting (under normal conditions) or a maximum of 24 hours. Leaving it overnight without a protective layer invites rust to return.
Mistake 3: Not checking the thickness of each paint layer. Paint too thin: insufficient protection. Paint too thick: internal cracking due to shrinkage. DFT (Dry Film Thickness) must be measured using an Elcometer or equivalent device.
Mistake 4: Not performing an Adhesion Test – A cross-cut test (ISO 2409) or pull-off test (ISO 4624) must be conducted to confirm the paint is firmly bonded to the substrate.
HAI LONG STEEL STRUCTURE IS A REPUTABLE STEEL STRUCTURE MANUFACTURER
Choosing the right steel structure fabrication partner is the deciding factor for the success of every construction project.
In a market context that increasingly demands high quality, schedule, and safety, Hai Long Steel Structure is proud to be a leading brand trusted by many major domestic and international investors:
- A team of experienced engineers, architects, and experts, knowledgeable in the design and construction of factory buildings and warehouses according to international standards.
- Diverse construction capabilities, meeting all project scales and requirements, from simple to complex factory buildings.
- Owning a steel structure factory with modern machinery and equipment systems, ensuring fast, accurate, and safe construction.
- Professional construction process, strictly managed.
Are you looking for a reputable and professional steel structure manufacturing and erection contractor?
Contact Hai Long Construction for free consultation and quotation:
- HAI LONG CONSTRUCTION JSC
- Address: Taiyo Building, 97 Bach Dang, Hong Bang Ward, Hai Phong City, Vietnam.
- Hotline: 084 6625 888
- Email: info@hailongjsc.vn
