What Is Fettling? A Guide to Fettling Galvanised Steel NSP Coatings

What Is Fettling? A Detailed Guide to Fettling Galvanised Steel

When steelwork is hot-dip galvanised, the resulting zinc coating does not always have the smooth, uniform appearance required for architectural metalwork.

Raised areas, zinc runs, sharp points and excess deposits can remain on the surface. If these imperfections are not addressed before painting or powder coating, they are likely to remain visible in the finished product.

This is where fettling becomes important.

Fettling galvanised steel is the process of removing or reducing surface imperfections so the item is safer to handle, fits correctly and has a more suitable surface for the specified coating system.

However, fettling must be carried out carefully. Removing too much material can reduce the thickness of the protective galvanised layer and potentially compromise the corrosion protection it was intended to provide.

At NSP Coatings, we regularly work with gates, railings, structural steelwork and architectural metalwork. We understand how the initial zinc protection method can affect the preparation, appearance and quality of the final finish.

What does fettling mean?
Why is galvanised steel fettled?
When do protective coating companies need to fettle steelwork?
What imperfections can be caused by hot-dip galvanising?
Is fettling needed on every metal item?
Does fettling improve coating adhesion?
Can fettling damage galvanised steel?
How can fettling be avoided?
Hot zinc spray compared with hot-dip galvanising
Choosing the right protective coating system

What Does Fettling Mean?

In its simplest form, fettling means removing surface imperfections from a manufactured or coated item.

The term can be used in several manufacturing industries, including metal casting, fabrication and protective coating.

When discussing galvanised steel, fettling normally refers to the mechanical preparation of the zinc-coated surface after the hot-dip galvanising process.

Handheld power tools such as grinders, orbital sanders and abrasive tools may be used to reduce high points, zinc runs, sharp edges and excessive build-up.

The objective is not to remove the galvanised coating. It is to improve the surface while retaining as much of the protective zinc layer as possible.

The exact level of fettling required will depend on the item, its intended use and the finish specified by the customer, architect or fabricator.

Industrial steelwork hidden from view may not require the same cosmetic standard as highly visible architectural gates or railings.

Why Is Galvanised Steel Fettled?

There are several reasons why a protective coating company may need to fettle galvanised steelwork before applying the next coating.

To Improve the Final Appearance

Surface imperfections are difficult to hide with paint or powder.

A primer and topcoat will follow the shape of the substrate beneath them. Therefore, a raised area or lump in the zinc is likely to remain visible after the final coating has been applied.

This is particularly important for:

Architectural gates
Decorative railings
Balustrades
Street furniture
External staircases
Public-facing steelwork
High-quality fabricated components

For these products, the appearance of the finished coating can be just as important as its protective performance.

To Remove Sharp or Unsafe Areas

Hot-dip galvanised steel can occasionally have sharp zinc points or rough deposits.

Fettling can reduce these areas and make components safer to handle, transport and install.

To Help Components Fit Together

Excess zinc around holes, edges, joints or connection points can cause assembly problems.

A build-up of material may prevent parts from fitting together correctly or interfere with bolts, hinges and other components.

Controlled fettling may be required to restore the necessary tolerances.

To Prepare the Surface for Further Coating

Galvanised steel is often subsequently painted or powder coated as part of a duplex protection system.

Before this happens, the surface must be properly assessed and prepared for the specified coating.

Fettling may form part of this preparation, although it should not be confused with the complete surface treatment required to achieve reliable coating adhesion.

When Do Protective Coating Companies Need to Fettle Steelwork?

Fettling is most commonly needed when items have already been hot-dip galvanised.

Hot-dip galvanising involves immersing suitably prepared steel into molten zinc. The zinc reacts with the steel and creates a protective coating.

Because the complete item is dipped into the zinc, the coating may collect in corners, hollow sections, drainage points and other areas, depending on the shape and design of the fabrication.

The amount of preparation required after galvanising can vary.

Some components may only require minor attention, while others may need more extensive work before they are suitable for a decorative coating.

A useful comparison is preparing painted woodwork. Applying more paint over an uneven surface will not make the surface smooth. The imperfections underneath must first be reduced.

The same principle applies to galvanised metalwork.

If a protective coater applies primer, paint or powder over raised zinc deposits, those defects can remain visible in the finished coating.

However, fettling must be controlled. Aggressive grinding can expose the underlying steel or reduce the zinc coating below the required level.

This is why the work should be completed by an experienced protective coatings company that understands both surface preparation and the specified corrosion protection system.

What Imperfections Can Be Caused by Hot-Dip Galvanising?

The appearance of a hot-dip galvanised surface can be influenced by the design of the component, its steel chemistry, drainage, venting and the galvanising process itself.

Typical areas that may require attention include:

Zinc runs or raised lines
Drips and sharp points
Excess zinc around holes
Rough areas or irregular deposits
Build-up around welds and corners
Ash or surface residues
Areas that interfere with assembly
Uneven sections that will remain visible after coating

Not every imperfection means the galvanising has failed. Some are primarily cosmetic.

However, they can still create difficulties when a smooth powder coated finish or decorative wet spray coating is required.

Is Fettling Needed on Every Metal Item?

No. Fettling is not automatically required on every metal component.

The need for it depends on the method used to apply the zinc protection.

One alternative to hot-dip galvanising is hot zinc spray, which is also known as thermal zinc spraying or zinc metal spraying.

Instead of immersing the steelwork in molten zinc, the zinc is melted and sprayed onto a carefully prepared steel surface.

The sprayed zinc particles build up to create a protective metallic layer.

Thermal spraying is used to apply zinc and aluminium coatings to steel structures for corrosion protection.

The bond is primarily mechanical, which means the cleanliness and surface profile of the steel are critical.

TWI explains that grit blasting is used to create the clean, suitably rough surface required by lower-energy thermal spraying processes.

At NSP Coatings, items receiving hot zinc spray are first shot blasted to Sa 3 standard, commonly described as blast cleaning to visually clean steel.

ISO 8501-1 provides the recognised system for visually assessing rust grades and preparation grades on steel surfaces.

This intensive surface preparation removes rust, mill scale and contamination while creating the profile needed for the sprayed zinc to adhere mechanically.

Because the zinc is sprayed onto the surface rather than collected as the item leaves a zinc bath, it can provide a more controlled surface for decorative coating systems.

It may still require a light manual rub-down before the next stage, but it does not usually create the same zinc runs, pooling or heavy build-up associated with hot-dip galvanising.

Does Fettling Improve Coating Adhesion?

Fettling can help remove raised areas and create a more consistent surface, but it should not be presented as the only requirement for coating adhesion.

Successful adhesion depends on the complete preparation and coating process, including:

The condition of the galvanised surface
Removal of contamination
The age of the galvanising
Surface roughness or profiling
Compatibility of the primer
Correct application conditions
Compliance with the coating manufacturer’s specification

Simply grinding or sanding a surface does not guarantee that a coating system will adhere correctly.

The coater must consider the substrate, the environment in which the item will be used and the complete coating specification.

With hot zinc spraying, adhesion of the metallic zinc coating is largely mechanical.

This is why the initial shot blasting stage is so important.

TWI notes that the adhesion of arc and flame-sprayed coatings depends on the substrate being extremely clean and suitably rough.

Can Fettling Damage Galvanised Steel?

Fettling can cause damage if it is too aggressive or completed without sufficient care.

The purpose is to reduce imperfections, not grind away the protective zinc layer.

Potential problems include:

Reducing the zinc coating thickness
Exposing the steel substrate
Creating inconsistent protection
Damaging edges or corners
Leaving deep grinding marks
Producing an uneven surface for subsequent coatings

Any exposed areas may require an approved repair system before further coatings are applied.

This is why fettling should be approached as a skilled preparation process rather than basic grinding work.

The operative must balance the desired appearance with the need to preserve corrosion protection.

How Can Fettling Be Avoided?

The best way to reduce fettling is to consider the complete protective coating system before the steelwork is galvanised.

Fabricators, architects and specifiers should think about the required final appearance, corrosion environment, component design and coating specification from the beginning of the project.

Where a highly controlled decorative finish is required, hot zinc spray may be a suitable alternative to hot-dip galvanising.

At NSP Coatings, the typical process can include:

Inspecting the fabricated steelwork
Removing fabrication defects where necessary
Shot blasting every item to the required preparation standard
Applying the specified thickness of thermally sprayed zinc
Completing a light surface preparation where required
Applying compatible primers and topcoats
Checking the coating thickness through our quality assurance process

This approach gives the protective coater greater control over the preparation and application stages.

It can also reduce the time spent correcting heavy zinc deposits before painting or powder coating.

The most appropriate system must still be selected according to the project specification.

Some projects specifically require hot-dip galvanising, while others may be better suited to hot zinc spray.

Hot Zinc Spray Compared With Hot-Dip Galvanising

Both processes use zinc to protect steel from corrosion, but they apply it in different ways.

Hot-dip galvanising coats the item by immersion in molten zinc.

It can provide protection to internal and external surfaces that the zinc can reach, making it suitable for many structural and industrial applications.

However, the immersion and withdrawal process can leave deposits that require fettling when a particularly smooth decorative finish is expected.

Hot zinc spray applies molten or semi-molten zinc particles directly to a shot-blasted steel surface.

Thermal spray processes are widely used to protect steel structures against atmospheric and aqueous corrosion, including large structures such as bridges and lamp posts.

Potential advantages of hot zinc spray for suitable projects include:

A controlled zinc application
No immersion of the complete fabrication
Reduced risk of zinc pooling and runs
Minimal heat input into the steelwork
A surface that can be prepared for paint or powder coating
Strong corrosion protection when used as part of the correct system
Suitability for large architectural and fabricated components

TWI notes that thermal spraying forms a coating with minimal heating of the substrate, helping to minimise component distortion.

Cost and lead-time comparisons should always be made against the complete project requirement.

Size, design, zinc thickness, preparation, transport and topcoat specification can all affect which solution is most economical.

Hot Zinc Spray Specialists in the South East

NSP Coatings provides hot zinc spray, shot blasting, powder coating and wet spray coatings for customers across London, Kent, Essex and the wider South East.

We use hot zinc spray extensively because it gives us close control over the preparation and coating process.

It is also an important part of selected systems covered by our long-term protective coating guarantee.

Our large facilities allow us to process gates, railings, structural steelwork and substantial fabricated components under one roof.

With dedicated blasting, zinc spraying, painting, powder coating and quality assurance teams, we can manage the complete coating process and maintain traceability throughout the project.

The right finish begins long before the final colour is applied.

Choosing the correct zinc protection and surface preparation method can reduce remedial work, improve appearance and support the long-term performance of the completed coating system.

View examples of completed projects in our Case Studies, learn more about Hot Zinc Spray and Grit Blasting, or contact NSP Coatings to discuss the correct protective system for your steelwork.

For the quickest answers, call our team on 01474 363719.

External References

TWI: Thermal spraying processes

TWI: Common applications of arc spraying

ISO: ISO 8501-1 steel surface preparation standards