15th October 2026
What Is Thermal Zinc Spraying?
Process, Benefits and Applications
Thermal zinc spraying protects steel from corrosion by applying a layer of molten zinc that creates a durable barrier and provides sacrificial protection to the metal beneath.
Corrosion can quickly damage the appearance of metalwork and, if left untreated, may eventually affect its strength and service life. Thermal zinc spraying, also known as hot zinc spray, zinc spraying or zinc arc spraying, is an effective way to protect steel and iron products from rust.
This guide explains how thermal zinc spraying works, its main advantages and how it compares with hot-dip galvanising.
What Is Thermal Zinc Spraying?
Thermal zinc spraying is a corrosion-protection process in which molten zinc is sprayed onto a properly prepared steel or iron surface.
The zinc cools almost immediately, creating a solid protective coating bonded to the substrate. This coating separates the steel from moisture and other environmental contaminants.
Zinc also provides sacrificial protection. If a small area of the coating becomes damaged, the surrounding zinc corrodes preferentially, helping to protect the exposed steel underneath.
Thermal zinc spraying is used across construction, manufacturing, infrastructure, architectural metalwork and marine applications.
How Does the Hot Zinc Spray Process Work?
A successful thermal zinc coating depends on careful surface preparation, controlled application and thorough quality checks.
1. Preparing the Metal by Grit Blasting
The steel is first cleaned by shot or grit blasting. This removes rust, mill scale and existing contaminants while creating the surface profile needed for the zinc coating to adhere correctly.
Good surface preparation is essential. Applying zinc to a poorly prepared surface can affect adhesion and reduce the performance of the complete coating system.
2. Melting the Zinc Wire
Two zinc wires are fed into an arc spray gun. An electrical arc generated between the wires melts the zinc at the tip of the gun.
Compressed air then atomises the molten zinc into fine particles.
3. Spraying the Zinc onto the Surface
The operator directs the zinc particles onto the prepared metal. The particles cool and solidify on contact, gradually building up a protective zinc layer.
The specified coating thickness should reflect the product’s intended use, exposure environment and required durability.
4. Applying a Sealer or Topcoat
The zinc layer can be sealed or covered with a compatible powder coating or wet-paint system.
Adding a topcoat provides the required colour and finish while creating a duplex coating system: the topcoat protects the zinc, and the zinc protects the steel.
What Are the Benefits of Thermal Zinc Spraying?
Thermal zinc spraying offers several important advantages:
- Long-term corrosion protection for steel and iron
- Sacrificial protection if the surface coating is locally damaged
- A relatively low heat input, reducing the risk of distortion
- Controlled coating thickness across the component
- No requirement to immerse the component in a molten zinc bath
- Suitability for large or complex items that may not fit inside a galvanising bath
- A consistent surface for compatible powder or wet-paint topcoats
- No galvanising drips or zinc pooling requiring extensive fettling
The process is particularly valuable when both corrosion protection and a high-quality decorative finish are required.
Where Is Hot Zinc Spray Used?
Thermal zinc spraying can protect a wide range of fabricated steel products, including:
- Balustrades and balconies
- Gates and railings
- Architectural metalwork
- Structural steelwork
- Bridges and platforms
- Street furniture
- Industrial frames and components
- Chassis and vehicle components
- Steelwork used in coastal or exposed environments
Because the process is applied using a spray gun, the operator must be able to reach the surface directly. It is therefore less suitable for inaccessible internal areas, such as the inside of long or sealed tubes.
Thermal Zinc Spraying vs Hot-Dip Galvanising
Thermal zinc spraying and hot-dip galvanising both use zinc to protect steel, but the application processes are different.
| Consideration | Thermal zinc spraying | Hot-dip galvanising |
|---|---|---|
| Application | Zinc is sprayed onto prepared steel | The item is immersed in molten zinc |
| Heat input | Relatively low | The complete component is exposed to bath temperature |
| Item size | Not restricted by the size of a galvanising bath | Limited by the available bath dimensions |
| Coating control | Thickness can be adjusted during spraying | Coating develops during immersion |
| Internal surfaces | Limited to areas the spray can reach | Can coat suitable internal surfaces when correctly vented and drained |
| Finish | Consistent sprayed finish suitable for compatible topcoats | May contain drips, lumps or rough areas that require fettling |
| Preparation for topcoating | Usually requires cleaning or light preparation | May require fettling and further preparation |
Hot-dip galvanising can be a practical choice for hollow products or components where internal coverage is important.
Thermal zinc spraying is often preferred for architectural steelwork, large fabrications and products that require a controlled surface before powder coating. The correct choice depends on the design, exposure conditions and required finish.
Can Thermal Zinc Spray Be Powder Coated?
Yes. Thermal zinc spray can form the protective layer beneath a compatible powder coating system.
The zinc coating provides corrosion protection, while the powder coating supplies the specified colour, texture and appearance. Correct sealing, preparation, application and curing are important to prevent coating defects and achieve the intended performance.
This combination is commonly used for external architectural metalwork, railings, balconies and other products where appearance and durability are equally important.
How Long Does Thermal Zinc Spraying Last?
The service life of a thermal zinc coating depends on:
- The zinc coating thickness
- The standard of surface preparation
- The surrounding environment
- The chosen sealer or topcoat
- The quality of application
- Inspection and quality-assurance procedures
- Future cleaning and maintenance
- Whether the completed coating becomes damaged
It is therefore important not to apply one lifespan claim to every hot zinc sprayed product.
NSP Coatings’ Ultra60 system carries a 60-year anti-corrosion guarantee for qualifying ferrous substrates. It is a complete, independently validated system involving SA3 shot blasting, hot zinc spray, specified powder-coating layers, thickness testing, quality control and documented maintenance conditions. The guarantee does not apply automatically to every hot zinc sprayed item. Read the full guarantee conditions.
What Should You Specify?
A thermal zinc spraying specification should identify more than the words “hot zinc spray”. It should consider:
- The steel substrate and component design
- The required surface-preparation standard
- The zinc coating thickness
- The environmental exposure category
- Whether a sealer, primer or topcoat is required
- The colour and finish
- Inspection and thickness-testing requirements
- Expected maintenance
- Any required guarantee or certification
Discussing these requirements before fabrication and coating helps avoid inaccessible surfaces, unsuitable details and unrealistic performance expectations.
Speak to NSP Coatings About Thermal Zinc Spraying
NSP Coatings provides shot blasting, thermal zinc spraying and powder coating from one facility, giving our team control over every stage of the protective coating process.
Whether you need a standalone zinc coating or a complete corrosion-protection system with a decorative powder-coated finish, we can help you select the appropriate preparation, coating thickness and topcoat for your project.
Contact NSP Coatings to discuss your metalwork and the environment in which it will be installed.
