Step-by-Step Epoxy Coating Inspection Procedure

By Pritam Singh, QC Coating Inspector

In my 11 years of experience in oil & gas coating inspection, I have seen that epoxy coating performance depends more on inspection control than on product quality. Even the best epoxy system can fail if surface preparation, mixing, application, or curing conditions are not properly monitored.

This article explains the complete step-by-step epoxy coating inspection procedure I follow on site — from surface preparation to final acceptance.

Why Epoxy Coating Inspection Is Critical

Epoxy coatings are widely used for:

• Structural steel
• Pipelines
• Tanks
• Offshore platforms
• Process piping

They provide excellent:

✔ Corrosion resistance
✔ Chemical resistance
✔ Mechanical durability

However, epoxy systems are sensitive to:

• Surface contamination
• Incorrect mixing ratio
• High humidity
• Improper curing conditions

Proper inspection ensures long-term coating performance.

Step 1: Documentation Review (Before Work Starts)

Before coating begins, I verify:

✔ Approved coating specification
✔ Project ITP (Inspection & Test Plan)
✔ Coating manufacturer Product Data Sheet (PDS)
✔ Material approval documents
✔ Batch numbers and expiry dates
✔ Calibration certificates for instruments

Never start coating without document verification.

Step 2: Surface Preparation Inspection

Epoxy performance depends heavily on surface preparation.

I check:

🔹 Cleanliness Grade

Typically:

• Sa 2½ (Near-White Metal) for most oil & gas projects
• Sa 3 for severe marine or immersion service

🔹 Surface Profile

Usually:

• 50–75 µm for standard epoxy
• 75–100 µm for high-build systems

Measured using replica tape or depth gauge.

🔹 Dust Level

Conduct dust test to ensure no loose contamination.

🔹 Soluble Salts

If required, perform chloride test to ensure within specified limits.

No primer application is allowed until surface preparation is accepted.

Step 3: Environmental Condition Monitoring

Before and during application, I record:

✔ Air temperature
✔ Steel surface temperature
✔ Relative humidity
✔ Dew point

Requirement:
Steel temperature must be at least 3°C above dew point.

High humidity can cause:

• Blushing
• Poor curing
• Adhesion failure

Environmental readings are recorded before coating and periodically during application.

Step 4: Paint Material Inspection

Before mixing epoxy, I verify:

✔ Correct material (base + hardener)
✔ Mixing ratio as per PDS
✔ Shelf life validity
✔ Proper storage conditions

Epoxy coatings are two-component systems. Incorrect mixing ratio leads to:

• Soft film
• Incomplete curing
• Early failure

Step 5: Mixing and Induction Time

Epoxy coatings require proper mixing:

✔ Use mechanical mixer
✔ Mix for recommended duration
✔ Observe induction time (if specified)

Induction time allows chemical reaction to start before application.

Skipping induction time may result in improper curing.

Step 6: Application Inspection

During application, I monitor:

✔ Spray technique
✔ Uniform coverage
✔ Wet Film Thickness (WFT)

WFT is checked to ensure final Dry Film Thickness (DFT) will meet specification.

Poor spray technique can cause:

• Runs and sags
• Pinholes
• Uneven thickness

Step 7: Dry Film Thickness (DFT) Measurement

After curing, DFT is measured using calibrated DFT gauge.

I follow:

✔ Minimum spot measurements
✔ Average thickness requirement
✔ Maximum thickness limits

Too low DFT:

• Reduced corrosion protection

Too high DFT:

• Cracking
• Solvent entrapment

Step 8: Holiday / Pinhole Testing (If Required)

For tanks or immersion service:

✔ Perform holiday test
✔ Check for coating discontinuities

Any defects must be repaired as per procedure.

Step 9: Visual Inspection

Final inspection includes checking for:

✔ Runs
✔ Sags
✔ Orange peel
✔ Dry spray
✔ Blistering
✔ Missed areas

Touch-up areas must follow approved repair procedure.

Step 10: Curing Verification

Epoxy curing depends on temperature.

I verify:

✔ Minimum curing time
✔ Hardness check (if required)
✔ Recoat window compliance

Applying next coat outside recoat window may require surface roughening.

Common Epoxy Coating Defects

• Amine blush
• Pinholes
• Cracking
• Delamination
• Solvent entrapment
• Dry spray

Most of these are caused by poor environmental control or incorrect application thickness.

My Practical Site Checklist

Before approving epoxy coating, I confirm:

✔ Surface preparation accepted
✔ Environmental conditions compliant
✔ Correct mixing ratio
✔ DFT within specification
✔ No visible defects
✔ All inspection records completed

Only then do I release the area.

Conclusion

Epoxy coating inspection is a systematic process — not just checking DFT at the end.

From my field experience, the most critical factors are:

1️⃣ Surface preparation quality
2️⃣ Environmental control
3️⃣ Correct mixing ratio
4️⃣ Proper thickness control

If these are managed properly, epoxy systems perform reliably for years.

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