The basics of rust protection through substrates, surface preparation and the environment

Introduction

When rust develops on coated metal, the coating material is often the first thing suspected.

“Why does it rust even though a rust-preventive coating is used?”
“Would switching to a stronger coating solve it?”
“Would it be better to change the coating manufacturer?”

This is a natural reaction.
However, rust on coated metal is not caused by coating materials alone.

Moisture or salts remained on the metal surface before coating.
Oil or rust-preventive oil was not adequately removed.
Rinsing or drying after surface preparation was insufficient.
Film thickness was insufficient.
Moisture remained in joints or gaps.
The product was used outdoors or in high humidity.

When such conditions combine, rust can occur even with a rust-preventive coating.

Selecting a coating material alone is insufficient to prevent rust on coated metal.
The substrate, surface preparation, coating conditions, drying, storage and service environment need to be considered together.

1. The role of the film in metal coating

A metal coating film does more than add color.

It protects the metal substrate from the external environment.
Particularly important is protection against moisture, oxygen, salts, chemicals, oil, heat, impact and other factors.

Moisture and oxygen are involved in metal rusting.
The presence of corrosion-promoting substances such as salts, acids or alkalis makes rust progress more readily.

The film is a protective layer that limits these external factors reaching the metal substrate.
However, it is not a perfect barrier.

Film thickness is insufficient.
There are pinholes.
The film is scratched.
Drying is insufficient.
Adhesion to the substrate is weak.
Contamination remained before coating.

Under these conditions, the film does not fully provide its protective function.
Rust may consequently progress beneath the film.

2. Why rust can occur even with rust-preventive coatings

When rust occurs despite a rust-preventive coating, the causes can be considered in five broad categories.

2-1. Causes of rust remain on the substrate before coating

Rust, salts, acids, alkalis, treatment solution, oil, rust-preventive oil, moisture or machining debris left on the metal surface before coating can become starting points for rust beneath the film.

Invisible substances may remain even when the surface appears clean.
Perspiration, salts and residual surface preparation chemicals in particular can readily cause rust later.

Even with a rust-preventive coating, rust may progress under the film if the substrate is not clean before coating.

2-2. Surface preparation is insufficient

Degreasing, cleaning, chemical conversion treatment, rinsing and drying are important steps in metal surface preparation.

Inadequate degreasing leaves oil that prevents adhesion.
Inadequate rinsing leaves treatment chemicals or salts.
Inadequate drying traps moisture under the film.

Surface preparation is the foundation of coating quality.
If it is inconsistent, rust problems can occur regardless of how good the coating material is.

2-3. Film thickness is insufficient

A coating film can provide protection only when it has sufficient thickness.

If it is too thin, moisture and oxygen can more readily reach the metal substrate.
Corners, edges, areas around holes, welds and jig contact points are also prone to insufficient thickness.

Even with sufficient thickness on flat areas, rust starts at edges or joints where thickness is insufficient.

When investigating rust, the film thickness at the rusty locations needs to be checked, not just the average thickness.

2-4. Drying and curing conditions are inappropriate

A film is not complete merely because coating has been applied.
It delivers its performance only after proper drying and curing.

Drying temperature is low.
Drying time is short.
Oven temperature varies.
The actual product temperature does not reach the specified temperature.
A thick coating delays internal drying.

These conditions can leave the film insufficiently cured, reducing water resistance or adhesion.
Moisture can then enter more readily and cause rust or blistering.

2-5. The service environment does not suit the coating specification

Indoor and outdoor products require different levels of rust protection.

Outdoor use.
High humidity.
Condensation.
A salt-containing coastal environment.
Chemical exposure.
Wetting with water.
Contact with oil or cleaning agents.
Heat or impact.
Abrasion or rubbing.

Such environments increase the load on the film.

Selecting a material without checking the service environment may result in rust after shipment or use, even if the coating initially looks satisfactory.

3. Why rust problems commonly occur in Vietnamese factories

Several factors in Vietnam increase the risk of rust on coated metal.

High temperature and humidity require particular attention.
Humidity rises during the rainy season, making condensation and residual moisture more likely.
Leaving products for a long time after surface preparation may allow moisture to return to the surface and cause renewed rust.

Some factories also face issues such as the following.

• Temporary storage after surface preparation is exposed to humidity
• Substrates are stored outdoors or in partly outdoor areas
• Rinse water management is inadequate
• Drying oven temperature control is inadequate
• Operators touch prepared metal with bare hands
• The cleanliness of gloves and wiping cloths is not managed
• Acceptance criteria for substrates before coating are unclear
• Records of rust causes are not retained

Rust problems do not necessarily arise from one major cause.
Small management shortcomings can combine and become apparent as rust after coating.

4. What to check first when rust appears

Before immediately changing the coating material, it is important to check the following items.

4-1. Where is the rust?

The rust location provides clues to its cause.

• Flat areas
• Edges
• Corners
• Around holes
• Welds
• Joints
• Jig contact points
• Areas scratched during handling
• Areas where water tends to collect

Frequent rust at edges or corners may relate to insufficient film thickness.
Frequent rust at joints or around holes may relate to residual moisture or treatment solution.

4-2. When did the rust appear?

The timing of rust appearance is also important.

Immediately after coating?
After drying?
Found during pre-shipment inspection?
After transport?
After use by the customer?
Does it increase only in the rainy season?

The timing helps distinguish a process problem from a storage, transport or service-environment problem.

4-3. What pattern does the rust have?

Rust has recognizable patterns.

Does it progress in filaments?
Does it appear as spots?
Does it run in lines?
Does it resemble fingerprints?
Does it spread from a joint?

The pattern is an entry point for root-cause analysis.
Rather than simply recording “rust,” it is important to take photographs and classify the pattern.

4-4. What was the substrate condition before coating?

Check the substrate condition before coating.

• Was rust already present?
• Was any oil left?
• Was rust-preventive oil properly removed?
• Was any moisture left?
• Was any machining debris or burr left?
• Were acids, alkalis or treatment solution left?
• Was the substrate touched with bare hands?

Even when rust becomes visible after coating, its cause may have remained from before coating.

4-5. Were surface preparation and drying consistent?

Check management of the surface preparation process as well.

• Degreasing solution concentration
• Chemical conversion bath concentration
• Rinse water cleanliness
• Water drainage condition
• Drying temperature
• Drying time
• Time between surface preparation and coating
• Storage location after surface preparation

Particular care is needed when the interval between surface preparation and coating is long.
In high humidity, the prepared surface may again be exposed to moisture or contamination.

5. Basic measures for preventing rust

Preventing rust on coated metal requires reviewing process control alongside material selection.

5-1. Clean the substrate before coating

The basic requirement is to leave no rust, oil, moisture, salts, treatment solution, machining debris or burrs.

It is particularly important not to touch metal with bare hands before coating.
Perspiration and salts are invisible but may cause rust later.

Using gloves is ineffective if the gloves are dirty.
Auxiliary items such as gloves, wiping cloths and air-blow equipment also need to be kept clean.

5-2. Thoroughly rinse and dry after surface preparation

Residual preparation chemicals can promote corrosion beneath the film.
Inadequate rinsing or drying leads to rust and blistering.

Holes, gaps, joints and pocket-shaped areas are particularly likely to retain moisture or treatment solution.
Checks need to cover structurally difficult-to-dry areas, not just visible surfaces.

5-3. Ensure the required film thickness

Appropriate film thickness is needed for rust protection.

However, thickness must not be judged only on flat areas.
Edges, corners, welds, areas around holes and jig contact points are prone to rust.

Film thickness measurement needs to check likely weak points, not only averages.

5-4. Select a coating specification suited to the service environment

Using an indoor coating specification unchanged outdoors or in high humidity creates a risk of early rust.

The primer, intermediate coat and topcoat system needs to be considered according to the service environment.
Omitting a primer may reduce cost, but it can increase future rust risk.

Coating specifications should be decided from the service environment and required durability, rather than price alone.

5-5. Include storage and transport conditions in quality management

Even with a correct coating process, moisture, scratches or condensation during storage or transport can cause rust.

Coated products are left in a humid environment.
Products are packaged before they have dried sufficiently.
They are scratched during transport.
They are stored where condensation occurs.

These conditions can also cause rust.

Rust protection of coated metal is not completed inside the coating booth alone.
It needs to be managed through storage, packaging and transport.

6. Questions to ask during customer visits

The following questions are useful when examining rust problems on coated metal.

For the factory manager

• Which products most commonly develop rust?
• Does rust appear immediately after coating or after shipment?
• Does it increase during the rainy season or periods of high humidity?
• Does rust cause recoating or delivery delays?

For production engineering

• Is the time between surface preparation and coating controlled?
• Are rinse water and drying conditions recorded?
• Is film thickness checked at edges and welds?
• Does the structure retain moisture in joints or holes?

For quality assurance

• Are rust locations and patterns recorded?
• Is the timing of rust appearance classified?
• Are there test criteria for film thickness, adhesion, humidity resistance and saltwater resistance?
• Can surface preparation conditions for defective products be traced?

For purchasing

• When buying steel with rust-preventive oil, is compatibility with the coating process checked?
• Is primer being readily omitted as a cost-reduction measure?
• Is the impact on coating quality checked when the substrate changes?

For coating shop-floor managers

• Is metal being touched with bare hands before coating?
• Are gloves and wiping cloths kept clean?
• Are rust, oil, moisture and contamination checked before coating?
• When rust appears, are photographs and locations recorded?

7. How CoatureVietnam can help

For rust problems on coated metal, CoatureVietnam can support checks of the entire process, as well as propose coating materials.

For example, we can support the following activities.

• Organizing the causes of rust problems
• Classifying photographs of defects
• Checking surface preparation conditions
• Reviewing film thickness measurement points
• Checking drying conditions
• Checking substrate storage conditions
• Preparing operator checklists
• Selecting rust-preventive coatings and primers
• Preparing improvement proposals for customers

What matters is more than selling coating materials.
It is working with customers to create processes that make rust less likely.

Conclusion

Coating materials alone cannot prevent rust on coated metal.

Rust-preventive coatings are important.
However, they are not sufficient on their own.

The substrate is clean before coating.
No surface preparation solution or moisture remains.
Sufficient film thickness is ensured.
Drying and curing conditions are appropriate.
The coating specification suits the service environment.
Storage and transport conditions are also managed.

Rust protection becomes consistent only when these conditions are all met.

When rust appears, check the substrate, surface preparation, film thickness, drying and environment in sequence, rather than immediately suspecting only the coating material.
This is the first step in reducing metal coating problems.

CoatureVietnam supports the prevention and improvement of rust problems by looking at the entire coating process, as well as material selection.

“Coating the Future with Science and Care”

Science means considering rust causes separately in terms of coating material, substrate, surface preparation, process and environment.
Care means creating clear standards and improvements so the shop floor does not keep facing the same problem.