Coating Steel Structures Serves More Than Appearance

Appearance matters in coating steel structures.

An attractive finish.
Consistent color.
No visible rust.
A clean impression as equipment or a building.

These affect the impression of a factory or facility.

However, appearance alone is not the most important objective.

The main objective is preventing corrosion and protecting safety.

Steel structures include steel frames, bridges, outdoor equipment, tanks, pipe supports, equipment support frames, plant members, port equipment, and factory-building members.

Once installed, these remain in use for long periods.
They may be exposed to rain or ultraviolet radiation outdoors.
Coastal locations may expose them to salt.
Inside factories, they may face oil, chemicals, moisture, heat, or dust.

Steel corrosion does more than impair appearance.

Progressive corrosion reduces strength.
It can cause equipment stoppages.
Repair costs increase.
Risks of falling parts, breakage, or accidents also rise.

Coating steel structures is therefore corrosion-protection design that safeguards safety.

Protective Coating Requires More Than Applying Rust-Preventive Paint

Steel-structure coating is sometimes assumed to require only applying rust-preventive paint.

Rust-preventive paints are certainly important.
However, a single coat may be insufficient for long-term corrosion protection.

Corrosion-protection performance depends on the following combination of factors.

• Surface preparation
• Primer
• Intermediate coat
• Topcoat
• Film thickness
• Recoating intervals
• Drying and curing conditions
• Application environment
• Prevention of transport damage
• On-site repairs
• Inspection and recoating plans

Protective coating therefore needs to be considered as a coating system, rather than an individual paint.

Even high-performance paint cannot perform as intended if rust or oil remains on the steel.
Insufficient thickness prevents adequate corrosion protection.
Unrepaired site damage becomes a starting point for corrosion.

Processes and management are as important as materials in protective coating.

Corrosion Directly Affects Structural Life and Safety

Steel corrodes under the effects of moisture and oxygen.

Salt, acidic gases, chemicals, humidity, and temperature changes further promote corrosion.

Progressive corrosion in steel structures causes the following problems.

• Loss of steel cross-sectional area
• Reduced strength
• Weakened bolts or welds
• Reduced equipment-support capacity
• Risk of falling members
• Equipment stoppages
• Higher repair costs
• Recoating or scaffolding costs
• Effects on customer or employee safety

Steel-structure corrosion is therefore a safety issue, as well as an appearance defect.

Assess long-term corrosion protection as well as initial appearance when coating steel structures.

Surface Preparation Is the Foundation of Corrosion Protection

Surface preparation is one of the most important protective-coating processes.

It prepares the steel surface before application to promote paint adhesion and corrosion-protection performance.

The following may be present on steel surfaces.

• Rust
• Mill scale
• Oil
• Dust
• Salt
• Moisture
• Weld spatter
• Existing coating film
• Dirt

Coating over these residues reduces film adhesion.
It also causes corrosion beneath the film.

Check steel surface condition before selecting protective paint.

Steel stored outdoors or in coastal areas particularly requires attention to rust or salt.

High-performance paint over insufficiently prepared surfaces will not provide the expected corrosion protection.

What Is Zinc-Rich Paint?

Zinc-rich paint is commonly used in protective coating for steel structures.

It is a protective paint containing a high proportion of zinc.

Zinc tends to corrode before iron.
Using this characteristic to protect iron is called sacrificial protection.

Simply put, zinc reacts first to suppress steel corrosion.

Zinc-rich paint is sometimes used as a primer on steel structures.
It is an effective option when long-term corrosion protection is required.

However, zinc-rich paint does not automatically ensure a long service life.

The following conditions are important for its performance.

• Appropriate surface preparation
• Specified film thickness
• Appropriate application method
• Drying and curing conditions
• Topcoat compatibility
• Recoating intervals
• Salt and oil removal

Zinc-rich paint is a powerful protective material, but poor application conditions reduce performance.

What Is MIO Paint?

MIO paint is also used in protective coating for steel structures.

MIO means micaceous iron oxide.
Plate-like pigments overlap within the film, making it harder for moisture or oxygen to reach the steel surface.

This function is called barrier performance.

MIO paint is often used as an intermediate coat.
The approach is to protect steel with the primer, delay moisture or oxygen ingress with the intermediate coat, and protect the film from ultraviolet radiation or rain with the topcoat.

MIO paint does more than provide color.
It forms the intermediate layer of a protective system, delaying the ingress of corrosive agents.

Each layer therefore has a role in protective coating.

The Roles of Primer, Intermediate Coat, and Topcoat

Consider each film layer separately in protective coatings for steel structures.

Primer

The primer contacts the steel directly.
Its main roles are rust protection and adhesion.

Zinc-rich paint or epoxy primers may be used.

An inadequate primer reduces corrosion protection across the entire coating system.

Intermediate Coat

The intermediate coat lies between the primer and topcoat.
Its main roles are achieving film thickness and providing barrier performance.

Materials such as MIO paint may be used to delay moisture or oxygen ingress.

The intermediate coat is extremely important for long-term corrosion protection.

Topcoat

The topcoat is the outermost layer.
Its main roles are weather resistance and appearance.

It protects underlying films from ultraviolet radiation, rain, temperature changes, and atmospheric contamination.

Topcoat deterioration makes damage to lower layers more likely.

Protective coatings therefore need to be considered through the functions of each layer, rather than one paint alone.

Film-Thickness Control Determines Corrosion Protection

Film-thickness control is extremely important in protective coatings.

Insufficient thickness allows moisture, oxygen, or salt to reach steel more readily.
This causes premature corrosion.

Excessive thickness also causes problems.

• Sagging
• Poor drying
• Insufficient curing
• Cracking
• Poor intercoat adhesion
• Higher costs
• Poor finish

The important point is more than simply applying a thick film.
It is consistently achieving the specified thickness in the required areas.

Insufficient thickness is particularly likely in the following locations.

• Edges
• Corners
• Welds
• Around bolts
• Around holes
• Recesses
• Rear surfaces
• Fixture-contact areas

Measuring only flat surfaces is insufficient for protective coatings.
Check vulnerable locations where corrosion tends to start.

Failure to Follow Recoating Intervals Reduces Protection

Protective coatings apply primer, intermediate coat, and topcoat in sequence.

Recoating intervals matter in this process.

Recoating too early may leave the underlying layer insufficiently dried or cured.
Applying the next layer then can cause retained solvent, blistering, poor drying, or poor intercoat adhesion.

Recoating too late may leave the underlying layer excessively cured or contaminated with dirt or moisture, reducing adhesion to the next layer.

Do not leave recoating intervals entirely to shop-floor judgment.

• Minimum recoating time
• Maximum recoating time
• Application temperature
• Humidity
• Drying condition
• Surface cleanliness

Check these and manage them as standard conditions.

Consider Factory Coating and On-Site Repairs Together

Steel-structure members may be coated in the factory.

Factory coating has the following benefits.

• Less affected by weather
• Equipment conditions are easier to control
• Working conditions are easier to establish
• Film thickness is easier to control
• Quality is easier to stabilize

However, factory coating alone does not complete quality assurance.

Members are transported from the factory to the site.
Films may be scratched during transport.
Cutting, drilling, welding, or bolting may take place on site.
Assembly may also scrape the film.

Leaving such damage or processed areas unrepaired allows corrosion to start there.

Consider factory coating and on-site repairs together for steel structures.

On-Site Repair Quality Determines Long-Term Corrosion Protection

On-site repairs may be seen as simple touch-up work.

However, they are extremely important for corrosion protection.

Common on-site repair problems include the following.

• Insufficient cleaning before repair
• Painting over rust or oil
• Using unspecified paints
• Insufficient film thickness
• Failure to follow drying times
• Inadequate recoating
• Repair areas that are too small
• No inspection after repair

Poor repair quality may turn supposedly repaired areas into corrosion starting points.

Standard work is therefore required for on-site repairs.

• Criteria for identifying areas needing repair
• Cleaning methods
• Rust-removal methods
• Paint to use
• Film thickness
• Drying time
• Inspection methods
• Recording methods

Treat on-site repairs as part of quality control for steel-structure corrosion protection.

Why Steel Structures Are Prone to Corrosion in Vietnam

Vietnam has many conditions that increase steel-structure corrosion risks.

The main factors are as follows.

• Heat and humidity
• The rainy season
• Coastal salt damage
• Outdoor storage
• Moisture inside factories
• Oil or chemicals
• Dust
• Damage at construction sites
• Delayed repairs
• Insufficient inspections

Heat, humidity, and salt particularly affect steel corrosion.

Factories near coasts or ports may require stricter protective specifications than ordinary outdoor environments.

During the rainy season, moisture or rust is more likely on steel surfaces before coating.
Drying and storage after coating also require attention.

When planning corrosion protection in Vietnam, check the local climate, storage, application, and repair arrangements rather than simply applying Japanese specifications unchanged.

Do Not Judge Long-Term Corrosion Protection by Initial Price Alone

Selecting steel-structure coatings by initial price alone can create substantial long-term costs.

An inexpensive specification may reduce initial costs.
However, premature corrosion incurs repair costs.
Recoating requires scaffolding or equipment downtime costs.
Corrosion-related safety risks create still greater problems.

Long-term corrosion protection requires considering the following costs and risks.

• Initial coating costs
• Repair costs
• Recoating costs
• Scaffolding costs
• Equipment downtime costs
• Inspection costs
• Accident risks
• Production-stoppage risks
• Effects on customer trust

A high-performance protective specification may appear expensive initially.
However, it may be advantageous over the long term if it extends recoating intervals, reduces repairs, or lowers safety risks.

Consider steel-structure coatings over their life cycle rather than initial price alone.

CoatureVietnam’s 4M Checks

CoatureVietnam checks protective coatings for steel structures through the 4M perspective.

Material: Materials

• Is zinc-rich paint necessary?
• Is an epoxy primer appropriate?
• Is an intermediate coat such as MIO paint necessary?
• Does the topcoat have weather resistance?
• Does the repair paint match the main specification?
• Can the system withstand coastal or chemical environments?
• Is it a manufacturer-recommended coating system?

Method: Methods

• Is the surface-preparation grade clearly defined?
• Is cleanliness checked before application?
• Is each layer’s thickness controlled?
• Are recoating intervals followed?
• Are drying and curing conditions appropriate?
• Are on-site repair procedures standardized?
• Are inspection and recoating intervals defined?

Machine: Equipment

• Is blasting equipment available?
• Is there space to coat large members?
• Can the application equipment handle high-build coating?
• Are ventilation and safety measures sufficient?
• Is drying and conditioning space available?
• Is a film-thickness gauge available?
• Are fixtures or packaging provided to prevent transport damage?

Man: People

• Do operators understand the purpose of protective coating?
• Do they understand the importance of surface preparation?
• Do they understand the roles of zinc-rich and MIO paints?
• Do they understand that insufficient thickness causes corrosion?
• Do they understand on-site repair criteria?
• Do supervisors share design life and service environment with the shop floor?

The Management perspective requires clarifying design life, warranty conditions, recoating intervals, corrosive environment, responsibilities, and repair plans.

Questions to Ask During Customer Visits

Sales and technical members can use the following questions for steel-structure protective-coating projects.

• Will the structure be used indoors or outdoors?
• Will it be used in a coastal or humid environment?
• How many years of durability are required?
• Are there corrosion-related safety risks?
• Are recoating or repair intervals defined?
• What surface-preparation method is used?
• Are zinc-rich or MIO paints planned?
• Are there film-thickness standards for each layer?
• Are recoating intervals controlled?
• Is there an on-site repair process after factory coating?
• How is damage during transport or construction managed?
• Is the coating specification being selected solely by initial price?

These questions identify issues across the protective system rather than paint selection alone.

Conclusion: Coating Steel Structures Protects Safety and Service Life

Coating steel structures is more than a process for improving appearance.

It is important technology for preventing corrosion, protecting structural safety, extending equipment life, and reducing repair costs.

Zinc-rich paint protects steel as a corrosion-protection primer.
MIO paint delays moisture or oxygen ingress as an intermediate coat.
The topcoat protects the film from ultraviolet radiation or rain.

However, these materials should not be considered individually.

Design them as a protective system encompassing surface preparation, thickness, recoating intervals, drying conditions, factory coating, on-site repairs, and inspection intervals.

CoatureVietnam treats steel-structure coating as improvement proposals supporting customer safety, equipment life, and maintenance costs, as well as the sale of rust-preventive paint.

Coating the Future with Science and Care

Putting this into practice means supporting protective-coating design and improvement suited to Vietnam’s heat and humidity, coastal environments, and actual construction-site conditions.