The fundamentals of coating quality from the workpiece material perspective

Introduction

When a coating problem occurs, the paint is often the first thing suspected.

“Is this paint at fault?” “Would changing to a better paint solve the problem?” “Why is rust appearing even though we use an anticorrosive paint?”

These questions often arise in coating operations.

In practice, however, coating problems are not caused by paint alone. Even with the same paint, changing the substrate being coated significantly changes both the problems that occur and the causes that need to be checked.

Are you coating metal? Stainless steel or aluminum? Plastic? Wood, glass, or paper?

Changing the workpiece, meaning the material being coated, changes the approach needed to stabilize coating quality.

This article organizes coating problems likely to occur with different substrates and summarizes the fundamental points to check before selecting a paint.

1. Coating quality is not determined by paint alone

Coating quality is not determined solely by paint performance.

In practice, quality is determined by a combination of factors such as the following.

• Substrate type
• Substrate surface condition
• Surface preparation condition
• Residual oil, moisture, and foreign matter
• Application method
• Film thickness
• Drying conditions
• Service environment
• Storage and transport conditions
• Operator handling

For example, when rust appears on a metal part, examining only the paint’s corrosion protection will not identify the cause.

Moisture or salt may have remained before coating. Surface preparation chemicals may not have been thoroughly rinsed away. Sweat or salt may have remained where an operator touched the part with bare hands. Moisture may have remained in joints or gaps.

Understanding coating problems correctly requires an assessment of the substrate and the whole process as well as the paint.

2. Investigate rust in metal coating through the substrate, surface preparation, and environment

Rust is the most typical problem in metal coating.

When rust appears, people tend to suspect inadequate anticorrosive paint performance. Paint selection is, of course, important. However, paint is not the only cause of rust.

The following factors can lead to rust in metal coating.

• Rust remaining before coating
• Residual oil or rust preventive oil
• Residual acid, alkali, or salt
• Residual surface preparation chemicals or rinse water
• Insufficient drying
• Insufficient film thickness
• Pinholes
• Moisture remaining in joints or gaps
• Contact between dissimilar metals
• Outdoor use, high humidity, condensation, or a coastal environment

It is important to examine the pattern of the rust.

Rust spreading like threads. Rust in the shape of fingerprints. Rust forming streaks. Rust appearing as spots. Rust emerging from joints.

The pattern of rust provides clues to its cause.

For example, fingerprint-shaped rust may result from sweat or salt left by bare-hand contact. Streaks of rust may be related to flow marks left by surface preparation chemicals or water. Rust at joints may result from moisture or treatment chemicals remaining in gaps.

In metal coating, it is important to identify causes through the shape, location, and timing of rust rather than treating all rust as the same problem.

3. Stainless steel and aluminum are not necessarily easy substrates to coat

Stainless steel resists rust. Aluminum is also lightweight and offers excellent corrosion resistance.

From a coating perspective, however, these materials are not necessarily straightforward.

Stainless steel has a stable oxide film on its surface. This improves corrosion resistance but can make the surface difficult for paint to adhere to.

Aluminum’s coatability also varies greatly with surface treatments such as anodizing, sealing, chromate treatment, and chromium-free treatment. Coating without checking the type of surface treatment can cause poor adhesion or appearance defects.

Applying specifications for iron directly to stainless steel or aluminum is therefore risky.

Check the following points.

• What is the substrate?
• What surface treatment has been applied?
• What is the purpose of coating?
• Have adhesion tests been performed?
• Is surface preparation appropriate?
• Are there environmental requirements?
• Are there requirements for chromium-free treatment or VOC reduction?

Nonferrous metal coating requires adhesion, surface preparation, and environmental requirements to be considered together with paint selection.

4. In magnesium coating, safety management is part of quality management

Magnesium is lightweight and offers useful functional properties. It is sometimes used in information technology equipment, precision parts, and automotive and motorcycle components.

However, magnesium requires care during surface preparation for coating.

Safety is particularly important. Magnesium in dust form presents an increased fire and explosion risk. Preparation methods such as sanding and blasting therefore require careful consideration of safety equipment, dust collection, exhaust, and fire prevention.

Excessively aggressive preparation can also cause dimensional changes, surface residues, and poor adhesion. Strong surface treatment intended to improve adhesion can instead create defects.

The following points need to be checked for magnesium coating.

• Alloy type
• Surface preparation method
• Dust and ignition risks
• Dimensional changes caused by acid pickling
• Residues after conversion coating
• Need for a primer
• Adhesion testing
• Operator safety

For magnesium coating, a process that can be operated safely must be designed before quality improvements are pursued.

5. Plastic coating begins by identifying the material

There is a first point to check in plastic coating.

It is the material name.

Even similar-looking resin parts can have very different coatability depending on whether they are ABS, PP, PC, PS, PA, PBT, FRP, SMC, BMC, or another material.

Typical problems in plastic coating include the following.

• Poor adhesion
• Cissing
• Blistering
• Solvent cracking
• Inadequate drying
• Deformation
• Dust contamination
• Uneven film thickness during electrostatic coating

Relevant causes include mold release agents, plasticizers, solvent effects, heat deflection temperature, internal molding stresses, and static electricity.

Mold release agents are particularly important. If the agents used during molding remain on the surface, paint will have difficulty adhering. Even a surface that looks clean can retain a thin oily film, causing cissing or peeling after coating.

Simply wiping with a strong solvent is not a solution. Depending on the resin, solvent can cause cracking, swelling, or whitening.

Before looking for a paint for plastic coating, check the material, molding, mold release agents, solvents, and drying conditions.

6. Conductive treatment and grounding are important in electrostatic coating of plastics

Electrostatic coating is relatively easy to use with metal parts because metals conduct electricity and are easy to ground.

Plastics, however, generally have low electrical conductivity. Applying electrostatic coating without additional measures can result in poor wraparound, unstable film thickness, or a lack of improvement in transfer efficiency.

Electrostatic coating of plastics requires measures such as the following.

• Use a conductive primer
• Use a conductive treatment liquid
• Ground through the fixture
• Modify the surface
• Control humidity and elapsed time after treatment

It is important to assess more than the electrostatic coating equipment alone. The conductivity of the resin surface, fixture grounding, surface preparation conditions, and the conductive primer’s film thickness and drying conditions must also be checked.

7. In wood coating, consider moisture content, expansion, and contraction

Unlike metals and resins, wood contains moisture. It also expands and contracts with humidity.

Moisture content control is therefore very important in wood coating.

Excessive moisture content makes warping, distortion, foaming, and cracking more likely after coating. Wood also contains air internally, which can expand during forced drying and cause bubbles in the coating film.

An excessively hard coating film can also be a problem. If wood moves as humidity changes and the film cannot follow that movement, cracking occurs.

Check the following points in wood coating.

• Wood type
• Moisture content
• Storage environment
• Combination of primer, intermediate coat, and topcoat
• Film thickness
• Whether excessive coating thickness has been applied
• Hardener mixing ratio
• Drying conditions
• Flash-off time

Producing a hard coating film is not the only answer in wood coating. The film must be designed to accommodate the substrate’s movement.

8. The firing process determines quality in glass coating

Glass coating requires a different approach from conventional coating.

For metals and wood, the main process is applying paint and drying or curing it. Glass coating materials, by contrast, contain inorganic pigments and flux; they are applied and fired at high temperature to fuse them to the glass itself.

Control of firing is therefore more important in glass coating than the application operation itself.

There are three fundamental stages.

1. Preheating
2. Firing
3. Slow cooling

Preheating must sufficiently decompose and volatilize the organic components. If this is inadequate, gas evolves during firing and causes pinholes or discoloration.

Firing melts the flux and fuses it to the glass. The temperature must, however, be controlled to avoid exceeding the softening point of the glass itself.

During slow cooling, it is important to avoid cooling too rapidly. Poor cooling conditions leave strain in the glass, leading to cracking and quality defects.

9. Absorption, drying, and humidity control are important in paper surface finishing

Paper surface finishing aims to add gloss, enhance appearance, and protect against dirt and scratches. Methods include coating, pressing, and lamination.

Paper, however, absorbs readily and is sensitive to humidity. Uneven gloss and gloss loss are therefore likely to occur.

Possible causes of uneven gloss include the following.

• Inappropriate application viscosity
• Roller marks
• Uneven absorption by the paper
• Blushing caused by residual solvent

Possible causes of gloss loss include the following.

• Insufficient drying
• Pressure marks during stacking
• Storage at high humidity
• Paper expansion
• Reduced surface smoothness

Quality cannot be assessed adequately by examining paper only immediately after finishing. The effects of humidity on the paper must be anticipated through storage, transport, and customer use.

10. Adapt checking points to the substrate

Preventing coating problems requires different checking points for each substrate.

For metals, check rust, surface preparation, moisture, salt, and film thickness. For stainless steel and aluminum, check oxide films, surface treatments, and adhesion. For magnesium, check safety and surface preparation. For plastics, check the material name, mold release agents, plasticizers, and solvent effects. For wood, check moisture content, expansion and contraction, and flash-off time. For glass, check preheating, firing, and slow cooling. For paper, check absorption, drying, and storage humidity.

The first step toward solving a coating problem is therefore not checking the paint name.

What are you coating?What properties does that material have?Are surface preparation, application, drying, and storage appropriate for that material?

It is important to start with these questions.

Summary

Coating problems are not caused by paint alone. Changing the workpiece material changes both the causes and the corrective measures.

Metals require one approach. Resins require another. Wood, glass, and paper also require their own approaches.

This is precisely the thinking behind CoatureVietnam’s principle, “Coating the Future with Science and Care.”

Science means analyzing coating problems through the substrate, surface preparation, paint, drying, equipment, work practices, and environment instead of dismissing them through intuition or arguments about blame.

Care means organizing checks clearly and implementing improvements in a form that operators, managers, and customers can understand and accept, so that the same failures are not repeated.

Before changing the paint, examine the substrate. This provides a more accurate understanding of the causes of coating problems.

CoatureVietnam supports coating process improvements that include not only paint selection but also substrates, surface preparation, application methods, drying conditions, and daily operations.