How to Avoid Over-Thickness Problems in Epoxy Coating

Epoxy coatings are widely used in oil & gas, marine, power plants, water tanks, and structural steel projects due to their excellent corrosion resistance and durability.

However, one of the most common field issues is over-thickness (excessive dry film thickness – DFT).

While many believe “more thickness means more protection,” excessive epoxy thickness can cause serious performance failures.

This guide explains:

• Why over-thickness happens
• Risks of excessive epoxy DFT
• How to prevent it
• Inspection best practices
• Field control strategies

What Is Over-Thickness in Epoxy Coating?

Over-thickness occurs when the applied coating exceeds the specified maximum Dry Film Thickness (DFT).

Example:

If the specification states:

300–400 microns total DFT

And the applied coating measures:

550–700 microns

That coating is over-thick.

Even if visually it looks good, internally it may fail.

Why Over-Thickness Is a Serious Problem

Excess epoxy thickness can cause:

1️⃣ Solvent Entrapment

Thick epoxy layers trap solvents during curing.
This leads to:

• Blistering
• Soft film
• Poor adhesion

2️⃣ Cracking & Internal Stress

Epoxy shrinks during curing.
Excess thickness increases internal stress, causing:

• Cracks
• Delamination
• Brittle failure

3️⃣ Intercoat Adhesion Failure

Thick coats may not cure properly before the next layer, reducing bonding strength.

4️⃣ Extended Cure Time

Over-thick epoxy takes longer to cure and may never fully harden in deep layers.

5️⃣ Inspection Rejection

Most project specifications define maximum DFT limits. Exceeding them may require costly repair.

Common Causes of Over-Thickness

Understanding the root cause helps prevent the problem.

🔹 1. Improper Spray Technique

• Slow gun movement
• Short spray distance
• Excess overlap

🔹 2. Incorrect Nozzle Size

Using a larger spray tip than recommended increases output and film build.

🔹 3. Poor Wet Film Thickness (WFT) Monitoring

Failure to check WFT during application leads to uncontrolled build-up.

🔹 4. Rework on Edges & Welds

Applicators often apply excessive stripe coating in weld areas.

🔹 5. Lack of DFT Monitoring Between Coats

Not measuring intermediate coats can result in cumulative over-thickness.

How to Avoid Over-Thickness in Epoxy Coating

Now let’s discuss practical prevention methods.

✅ 1. Follow the Specification Strictly

Always verify:

• Minimum DFT
• Maximum DFT
• Recommended WFT
• Recoat window

Never assume more thickness equals better protection.

✅ 2. Control Wet Film Thickness (WFT)

WFT control is the most effective preventive measure.

Use Wet Film Gauge During Application

Check WFT:

• Immediately after spraying
• At multiple locations
• Especially on flat areas

Formula:

WFT = DFT ÷ % Volume Solids

Example:
If epoxy is 75% solids and required DFT is 300 microns:

WFT = 300 ÷ 0.75 = 400 microns

This prevents over-application before curing.

✅ 3. Use Correct Spray Equipment

• Follow manufacturer’s recommended tip size
• Maintain correct spray pressure
• Maintain proper spray distance (usually 30–45 cm)

Oversized spray tips are a common reason for high DFT.

✅ 4. Apply Multiple Thin Coats Instead of One Thick Coat

Instead of applying:

500 microns in one pass

Apply:

2 × 250 microns

Thin coats cure better and reduce solvent entrapment risk.

✅ 5. Monitor DFT After Each Coat

Use calibrated DFT gauge after curing of each coat.

If intermediate coat exceeds limit, adjust next coat thickness.

✅ 6. Control Stripe Coating

Edges and welds often receive excessive build.

Best practice:

• Apply controlled stripe coat
• Avoid repeated heavy brush application
• Measure DFT on edges

✅ 7. Maintain Environmental Conditions

High humidity and low temperature slow curing.

This increases risk of:

• Sagging
• Over-build
• Poor solvent release

Always monitor:

• Surface temperature
• Air temperature
• Relative humidity
• Dew point

Inspection Best Practices for Avoiding Over-Thickness

As a coating inspector, you should:

✔ Verify spray tip size
✔ Check WFT during application
✔ Record DFT per coat
✔ Compare readings with specification limits
✔ Monitor recoat intervals
✔ Educate applicators

Early detection prevents expensive repairs.

What If Over-Thickness Is Already Applied?

If DFT exceeds specification:

Minor Excess (Within tolerance)

Consult client/engineer — may be acceptable.

Moderate Excess

Allow extended curing and test adhesion.

Severe Over-Thickness

May require:

• Sanding down
• Mechanical grinding
• Full removal and recoating

Never ignore excessive thickness.

Key Takeaway

Over-thickness in epoxy coating:

❌ Does NOT increase protection
❌ Can cause internal failure
❌ Leads to rejection and rework

Proper WFT control, correct spray technique, and continuous inspection are the best preventive measures.

Frequently Asked Questions (FAQ)