Shot Blasting & Anti-Rust Coating for Steel Structures

One of the most common mistakes in steel structure construction—and the most costly to fix—is improper surface treatment of the steel before painting.

Many developers are drawn to the beautiful, glossy paint finish after completion, failing to realize that if the underlying steel surface isn't properly cleaned, the paint will peel and rust will begin to spread within just 12–24 months—no matter how expensive the paint used.

This is essential knowledge for all investors and engineers supervising steel structure projects.

Why do steel surfaces need to be cleaned before painting?

Hot-rolled steel from the mill always has a layer of grayish-blue iron oxide (mill scale) firmly adhering to its surface. This layer:

Prevent paint from adhering to the steel surface.
There is a different electrochemical potential than the steel below → creating an electrochemical cell → accelerating corrosion.
Brittle and easily peeled when steel deforms → pulling the paint layer off with it.

Besides mill scale, steel surfaces also contain rust (from storage), grease, dirt, and welding slag—all enemies of the protective coating.

What is shot blasting? Why not sanding or sandblasting?

Shot blasting (cleaning with steel balls) This is the process of blasting steel particles (steel shot or steel grit) at high velocity (60–80 m/s) onto a steel surface using a centrifuge, creating a strong mechanical impact to:

Completely remove mill scale, rust, and dirt.
Roughening the surface (anchor profile) helps the paint adhere better.
Achieve uniform cleanliness across the entire surface area of the component.

Comparing cleaning methods:

Method	Cleanliness achieved	Treatment cost	Speed	Popular with large KCT
Hand sanding	St 2 (low)	Very low	Very slow	No
Remove rust using a grinding machine.	St 3	Low	Slow	No
Sandblasting	Sa 2 – Sa 3	Medium	Medium	Limit (pollution)
Shot blasting	Sa 2.5 – Sa 3	Medium	Fastest	Industry standards

When you see in the technical specifications the requirement "clean to Sa 2.5", this is the cleanliness level according to ISO 8501-1 (SIS 055900) standard:

Step 1: Gentle cleaning — remove any loosely adhering materials. Not sufficient for protective coating.
Step 2: Thorough cleaning — remove most of the mill scale and rust. Some dullness remains.
Sa 2.5: Very thorough cleaning — virtually 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 coatings.
Sa 3: Cleaning to white metal — 100% removal. Requires extremely corrosive environments.

Surface roughness – the second equally important factor

Shot blasting not only cleans but also creates a rough surface—a microstructure of mountains and valleys on the steel—that helps the paint “root” itself into the surface instead of just sticking to the surface.

Commonly required surface roughness: Rz = 40–70 micrometers (measured by profilometer or compared with a Testex Press-O-Film standard sample).

If the roughness is too low (the surface is too flat): the paint will not adhere properly. If the roughness is too high (the sharp points are too high): the rough points may penetrate the thin layer of paint, creating localized corrosion spots.

Steel structure painting system

Protective coating for steel structures is not a single layer — but rather multi-layer system Each layer has its own function:

Layer 1: Anti-rust primer

Function: Adheres to steel surfaces and prevents moisture penetration.

Common type:

Zinc-rich epoxy primer: Zinc acts as a sacrificial anode to protect steel. This is a standard for FDI projects.
Alkyd primer: Lower cost, suitable for less corrosive environments.

Dry film thickness: 60–80 microns (measured with a dry film thickness gauge)

Layer 2: Intermediate/Tie Coat

Function: Increases the thickness of the paint system, improving moisture resistance.

Common type: Epoxy mastic — Good adhesion, moisture resistant, chemical resistant.

Dry film thickness: 75–100 microns

Layer 3: Topcoat

Functions: UV protection, atmospheric corrosion resistance, and aesthetic color and surface finish.

Common type:

Polyurethane topcoat: Beautiful, UV-resistant, and abrasion-resistant. Suitable for outdoor structures.
Acrylic polyurethane: Excellent colorfastness and UV protection.

Dry film thickness: 50–75 microns

Total thickness of the standard paint system at FDI factories: 180–255 microns (DFT — Dry Film Thickness)

Mistakes to avoid

Mistake 1: Do not paint when the surface is still damp – Relative humidity >85% or steel surface temperature below the dew point by 3°C → the paint will not cure properly and blistering is likely.

Mistake 2: Too much time between shot blasting and priming. Steel after shot blasting must be primed within 4–6 hours (under normal conditions) or a maximum of 24 hours. Leaving it overnight without a protective layer invites rust back.

Mistake 3: Do not check the thickness of each coat of paint. Paint too thin: insufficient protection. Paint too thick: internal cracking due to shrinkage. The Dry Film Thickness (DFT) must be measured using an Elcometer or equivalent device.

Mistake 4: Adhesion Test – A cross-cut test (ISO 2409) or pull-off test (ISO 4624) must be performed to confirm that the paint adheres firmly to the substrate.

HAI LONG STEEL STRUCTURES IS A REPUTABLE MANUFACTURER OF STEEL BUILDING STRUCTURES.

Choosing the right steel structure fabrication partner is a crucial factor in the success of every construction project.

In a market that increasingly demands quality, timeliness, and safety, Hai Long Steel Structure Proud to be the leading brand trusted by many large domestic and foreign investors:

Team experienced engineers, architects and experts, knowledgeable about the design and construction of warehouse factories according to international standards.
Diverse construction capacity, meeting all project sizes and requirements, from simple to complex factories.
We own a steel structure factory with a modern system of machinery and equipment., ensuring fast, accurate and safe construction.
Professional construction process, is strictly managed.

You are looking for one Prestigious and professional steel structure manufacturing and erection unit?

Contact Hai Long Construction for free consultation and quotation: