Common Sandblasting Defects and How to Avoid Them

By Pritam Singh, QC Coating Inspector

In my 11 years of coating inspection experience in oil & gas projects, I have seen that most coating failures originate from improper sandblasting. Even when steel “looks clean,” hidden defects such as wrong profile, contamination, or incomplete cleaning can lead to premature failure.

Sandblasting is not just cleaning — it creates the mechanical anchor profile required for coating adhesion.

1️⃣ Types of Surface Cleanliness (Blast Cleaning Standards)

Surface cleanliness is classified according to international standards such as:

• ISO (ISO 8501-1)
• AMPP (formerly SSPC/NACE)
• ASTM International

Below are the most commonly specified blast cleanliness grades in oil & gas projects:

🔹 Sa 1 – Light Blast Cleaning

Loose mill scale, rust, and paint removed. Not suitable for protective coating systems.

🔹 Sa 2 – Thorough Blast Cleaning

Most mill scale, rust, and coating removed. Some staining may remain.

🔹 Sa 2½ – Very Thorough Blast Cleaning (Near-White Metal)

Almost all mill scale, rust, and coatings removed. Only slight shadows or streaks allowed.

👉 Most common requirement in oil & gas projects.

Equivalent: SSPC-SP10 / NACE No.2

🔹 Sa 3 – White Metal Blast Cleaning

Complete removal of all visible contamination. Uniform metallic appearance.

Equivalent: SSPC-SP5 / NACE No.1

👉 Used for severe marine or immersion service.

2️⃣ Types of Surface Roughness (Surface Profile Classification)

Surface roughness (anchor profile) is the peak-to-valley height created during blasting. It provides mechanical bonding for coatings.

Standards reference:

• ISO 8503 series
• ASTM D4417

Surface profile is generally classified as:

🔹 Fine Profile

Approx. 25–50 µm (1–2 mils)

Used for:

• Thin coatings
• Shop primers
• Light-duty systems

🔹 Medium Profile

Approx. 50–75 µm (2–3 mils)

Most common for:

• Epoxy coatings
• General oil & gas structures

🔹 Coarse Profile

Approx. 75–125 µm (3–5 mils)

Used for:

• Heavy-duty coatings
• Offshore structures
• High-build epoxy systems

🔹 Extra Coarse Profile

Above 125 µm

Rarely required unless specified. Can cause:

• Excessive paint consumption
• Pinholes
• Peak rusting

Common Sandblasting Defects

Now that we understand cleanliness and profile types, let’s review the most common defects.

3️⃣ Inadequate Cleanliness

Problem

Surface does not meet required Sa grade (e.g., Sa 2 instead of Sa 2½).

Causes

• Insufficient blasting time
• Low pressure
• Worn nozzle
• Inexperienced operator

Prevention

✔ Compare with ISO visual reference standards
✔ Ensure correct pressure at nozzle
✔ Replace worn nozzles

4️⃣ Incorrect Surface Profile

A) Profile Too Low

Causes:

• Fine abrasive
• Low pressure
• Excessive recycling

Result:

• Poor adhesion
• Coating peeling

Prevention:
✔ Select correct abrasive size
✔ Verify profile using replica tape
✔ Adjust blasting parameters

B) Profile Too High

Causes:

• Coarse abrasive
• Excessive pressure

Result:

• Pinholes
• High paint usage
• Early corrosion at peaks

Prevention:
✔ Adjust abrasive type
✔ Confirm coating tolerance

5️⃣ Flash Rusting

Occurs when blasted steel is exposed to humidity.

Types:

• Light flash rust
• Moderate flash rust
• Heavy flash rust

Prevention:
✔ Monitor dew point (Steel temp ≥ 3°C above dew point)
✔ Prime within allowed window
✔ Control humidity

6️⃣ Dust Contamination

Dust left on surface reduces adhesion.

Prevention:
✔ Perform dust test
✔ Blow down with clean dry air
✔ Inspect under adequate lighting

7️⃣ Oil / Moisture Contamination

From compressed air system.

Prevention:
✔ Perform blotter test
✔ Maintain moisture separators
✔ Drain compressors regularly

8️⃣ Embedded Abrasive

Occurs when particles are trapped in soft steel.

Prevention:
✔ Use approved abrasive
✔ Maintain correct blast angle
✔ Conduct close visual inspection

Practical Field Acceptance Criteria

Before approving blasted surface, I verify:

✔ Cleanliness grade (Sa level with help of ISO 8501-1:Visual assessment of surface cleanliness)
✔ Surface profile (minimum 3 readings per location)
✔ Dust level
✔ Salt contamination (if required)
✔ Environmental conditions (with help of Due Point Meter)
✔ Air cleanliness (With help of blotter test)

Only after all parameters comply with specification do I release for coating.

For a detailed step-by-step inspection process, read my complete guide: Complete Surface Preparation Inspection Checklist for Oil & Gas Projects.

Conclusion

Sandblasting quality depends on two critical parameters:

1️⃣ Correct cleanliness grade
2️⃣ Correct surface profile range

If either is incorrect, coating life will reduce significantly.

From my experience, 80% of coating failures are directly related to surface preparation defects that were not properly inspected.

FAQ