Paint Blistering on Pre-Painted Steel? Your Complete Guide to Causes, Risks, and Solutions

Blistering appears as one or more dome-shaped bubbles on the coating surface. These bubbles can be different sizes. Inside the bubbles, there is usually a liquid (like water), a gas (like solvent vapor or air), or both.

Blistering means the coating has lost its bond, or adhesion, to the steel underneath. This can be a partial or complete loss of adhesion.

Blistering is one of the most serious forms of coating failure. When it appears, the protective function of that area is completely gone.

Part 1: The Core Reason for PPG/PPGL Blistering – Loss of Adhesion

The direct cause of a blister is pressure building up under the paint film. This pressure pushes the paint up. Finding the source of this pressure is key to finding the root cause. There are three main culprits.

Culprit 1: Poor Surface Preparation

This is the most common reason for paint blistering. The coating is like a blanket on the steel. If the bed is not clean, the blanket will not stay in place.

Soluble Salt Contamination: This is a hidden killer. Salts can come from cleaning chemicals or the industrial environment. They may be invisible on the steel surface. But these salts create osmotic pressure. When there is moisture in the air, the salts pull water through the paint film. This water collects under the coating, builds pressure, and causes osmotic blistering.

Oil, Grease, or Dust Residue: If the steel is not cleaned properly, oil or dust can remain on the surface. This blocks the bond between the paint and the steel. This creates weak spots where blisters can easily form when the temperature or humidity changes.

Substrate Corrosion: If the steel already has some minor rust (like white rust on galvanized steel) before painting, it creates a weak point for adhesion. This rust also absorbs moisture, which can cause blisters and faster corrosion later.

Culprit 2: Loss of Process & Environmental Control

Good process control is essential for quality. A small mistake can lead to big problems. Trapped Moisture or Solvents:

Incomplete Drying: The steel is not fully dried after cleaning and before painting.

Trapped Solvents: The paint contains solvents. If these solvents do not evaporate before the paint cures, they get trapped inside. When the panel gets hot (e.g., in the sun), the trapped liquid turns to vapor and expands, pushing up the paint to form a blister.

Coating Too Thick: If the paint is applied too thick in one go, the surface dries too fast. This traps solvents inside.

High Ambient Humidity: If the air is too humid during painting, moisture can get onto the steel surface or into the wet paint.

Contaminated Compressed Air: If the compressed air used for cleaning or spraying contains oil or water, it puts contaminants directly onto the surface.

Culprit 3: Issues with Raw Materials

• Paint Formulation Issues:

Moisture in Pigments: Some pigments or fillers in the paint can absorb moisture.

Improper Formula: The solvents in the paint may have the wrong boiling points, causing them to evaporate unevenly.

• Substrate Issues:

Improper Passivation: The passivation layer on galvanized steel helps with adhesion. If this layer is of poor quality or does not match the paint, it can cause adhesion problems.

Porous or Rough Substrate: A very rough surface can trap air in small pockets. This air can expand during heating and cause blisters.

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Part 2: The Serious Dangers of Blistering – From Skin Problem to Cancer

Blistering is not just a cosmetic problem. It is the beginning of a total system failure. The danger is progressive and fatal to the product.

• Complete Loss of Protection: In a blistered area, the coating is separated from the steel. This means all protection against corrosion and UV rays is gone.
• Irreversible and Accelerated Corrosion: When a blister breaks, the steel is exposed directly to the environment. Rust will form very quickly. Even if the blister does not break, the moisture and salts inside create a "mini corrosion cell." This causes corrosion to spread under the paint.
• Chain Reaction and Large-Scale Peeling: A single blister creates a path for more moisture to get in. This can cause more blisters to form nearby. Eventually, this can lead to large areas of the coating peeling off.
• Huge Repair Costs & Reputation Damage: It is almost impossible to properly fix blistering. Sanding and repainting a small area does not fix the root cause. The only real solution is often to replace everything. This leads to huge financial losses and a loss of customer trust.

Part 3: Standard Procedure for Handling Blisters Found During Production

Step 1: Immediate Actions – Control the Damage

When an operator or inspector finds blisters, they must act immediately. The goal is to stop making more defects and to preserve the evidence.

Stop the Line Immediately: This is the first and most important step. Continuing to run will only create more scrap and increase losses.

Isolate and Tag:

• Isolate the Coil: Remove the problem coil from the line or isolate it where it is.
• Tag Clearly: Use a bright tag like "Non-Conforming," "On Hold," or "Quality Issue." The tag must include:

    Date and time found

    Shift and operator name

    Problem description (e.g., "Surface Blistering")

    Coil ID number

    Approximate start and end meter marks of the defect

Sample and Record Information:

• Take Samples: Cut samples (at least 30cm x 30cm) from the beginning, middle, and end of the defect area. Label them clearly.
• Take Photos/Videos: Use a good camera to take close-up and wide shots of the blisters.
• Record Key Parameters: Immediately write down all key production settings at the time of the shutdown. Create a "Problem Report" with:
• Production Info: Line speed, tension.
• Coating Info: Paint batch number, viscosity, roller pressure, coating thickness.
• Curing Info: Oven temperature settings.
• Pre-treatment Info: Tank temperatures, concentrations, spray pressures.
• Raw Material Info: Steel coil batch number.

Step 2: Root Cause Analysis – Find the Real Culprit

After the immediate actions, a team from Quality, Technical, Production, and Maintenance should start a root cause analysis.

A Fishbone Diagram is a good tool for this.

Analysis Areas:

• Substrate (Steel):

Was it a bad batch of steel? Check the Mill Test Certificate (MTC). Look for contamination or white rust.

Did it get dirty before painting? Check for oil or dust drips in the uncoiling area.

• Pre-treatment:

Poor Cleaning? Check the cleaning tank's temperature and concentration.

Poor Rinsing? Are the water rinse tanks clean? Are spray nozzles clogged?

Salt Residue? (Key Suspect) Is the conductivity of the final pure water rinse too high?

Incomplete Drying? Is the drying oven hot enough? Is there still moisture on the steel?

• Coating Process & Environment:

Bad Paint? Is it a new batch of paint? Check the paint's test report (CoA). Contact the paint supplier.

Contaminated Paint? Are the pipes and storage tanks clean?

Trapped Solvents? (Key Suspect) Was there enough "flash-off" time for solvents to evaporate?

Bad Environment? Is the coating room too hot or humid? Is there a source of contamination nearby?

• Curing (Baking):

Heating Too Fast? Did the oven heat up too quickly, trapping solvents?

Uneven or High Heat? Did the oven temperature get out of control?

• Equipment:

Contaminated Air? Does the compressed air have oil or water in it?

Dirty Rollers? Are the rollers clean?

Step 3: Corrective & Preventive Actions

Based on the analysis, take action.

Corrective Actions (The Quick Fix):

• Targeted Fix: If the cleaning solution is bad, change it. If the paint is bad, switch to a new batch.
• Test Run: After fixing the issue, run a small test batch. Do not start full production until you are sure the problem is solved.

Preventive Actions (The Long-Term Fix):

• Update SOPs: Write the correct process settings into the Standard Operating Procedures (SOPs).
• Improve Maintenance: Add more frequent checks for key equipment (like air filters) to the maintenance schedule.
• Improve IQC: Create stricter inspection rules for incoming raw materials.
• Train Staff: Train employees on the causes of blistering and how to prevent it.

Step 4: Handling Affected Products

Full Inspection:

• Trace Back: Check all coils produced before the problem was found, especially those using the same raw materials.
• Isolate Forward: All products manufactured after the issue was identified should be considered "suspect" and require additional inspection until the problem is confirmed to be resolved.

Final Decision:

• The Rule: For any coils with confirmed blistering, the only correct action is to scrap them.
• The Reason: Blistering is a fatal, irreversible defect. Selling it, even at a discount, will destroy the factory's reputation.
• The Action: Scrapped coils should be physically destroyed (e.g., cut up) to prevent them from being used by accident. Keep detailed records of all scrapped material.

Part 4: How to Prevent Blistering from the Start – A Supplier's Duty

Prevention is the only way to solve the blistering problem. A reliable supplier must be excellent in these areas:

• Rigorous Surface Pre-treatment: This is the most important step. A supplier must have a complete multi-stage pre-treatment line. This includes double degreasing, thorough rinsing, a pure water rinse (to remove salts), and a high-quality chemical conversion coating. They must ensure every steel sheet is spotless and dry before painting.
• Precise Process Control: They must strictly control paint viscosity, coating thickness, flash-off time, and the curing curve (temperature and time).
• Clean Production Environment: The coating room must be a clean, climate-controlled space. The air filtration system must work well to keep contaminants off the wet paint.
• Strict Raw Material Inspection: They must inspect every batch of steel and paint to ensure they meet quality standards.

Paint blistering on pre-painted steel is a direct sign of serious flaws in a supplier's quality control system. It is not just one small defect; it is often the tip of a much larger iceberg.

As a buyer, when choosing a supplier, consider not just the price and color. You should inquire about their pre-treatment process, the cleanliness of their facility, and the rigor of their quality management system.

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