One is molten metal, the other is paint
Thermal spray zinc — usually called TSA, for thermal sprayed aluminium, or zinc arc spray — melts zinc wire in an arc and throws the molten droplets at the steel at high velocity. There is no binder, no solvent and no cure. The coating forms when the droplets flatten, freeze and lock onto a roughened surface. The result is a metallic zinc layer with the same chemistry as the wire it came from.
Cold spray zinc is a single-component coating: zinc dust, a small amount of organic binder, and solvent. It dries and cures like a paint. The zinc particles are in electrical contact with each other and with the steel, which is what makes the film sacrificial, but the film is a composite — metal particles in a polymer matrix, not a continuous metal layer.
That distinction explains almost every practical difference that follows: how thick it goes on, how porous it is, how it fails, and what it costs to apply.
Zinc mass: the 100% against 96% comparison is a distraction
Suppliers on both sides quote purity. It is the least important number in the comparison. What protects steel is the mass of zinc per square metre, because sacrificial protection is consumed in proportion to the current it has to supply.
TSA is typically specified at 100–150 µm. Zinc has a density of 7.14 g/cm³, so 120 µm of pure zinc is roughly 860 g/m². A cold spray zinc at 80 µm dry film, 96% zinc by weight and a dry film density around 4.2 g/cm³, carries about 320 g/m² of zinc. Same nominal “zinc coating”, less than half the zinc.
This is not an argument against cold spray zinc — it is an argument against comparing the two by film thickness. A 120 µm cold spray zinc film is not the equivalent of a 120 µm TSA film, and a specification written that way will under-protect the structure while looking generous on paper.
Porosity, sealing and how each one fails
TSA is porous. Depending on the process and the spray distance, porosity runs from about 5% to 15%. A sealed TSA system — zinc plus a sealer or a full paint system — is the only version that performs in a C4 or C5 environment. Unsealed TSA in a chloride atmosphere starts to show white rust and then red rust at the porosity, and the failure is not visible until it is advanced.
Cold spray zinc has no interconnected porosity to speak of, because the binder fills the space between particles. It goes on as a continuous film with a defined dry film thickness. The trade-off is temperature and abrasion: the organic binder limits continuous service temperature to roughly 120–150 °C, and the film is softer than a metallic zinc layer under sliding abrasion.
Failure modes differ accordingly. TSA fails at pores and at edges if the sealer is incomplete. Cold spray zinc fails at the interface if surface preparation was inadequate, or at the topcoat if the recoat window was missed — which is why the application data sheet matters as much as the zinc content.
Who can actually get on site
TSA is a plant or a well-equipped site operation. It needs a compressor with adequate volume, a arc spray unit, zinc wire, a blasting setup to reach Sa 2½ or better, and operators who do it often enough to keep the parameters stable. It is fast on large flat areas and it is the right answer on a new fabrication yard where the steel is on a bench and the blasting booth is next door.
It is the wrong answer in most of the situations that generate enquiries: a transmission tower that is already standing, a bridge bearing that has to be repaired in a two-hour possession, 3 mm steel that will distort under the heat input, a weld seam touched up after erection, or a galvanized member where the coating was damaged in transit. In those cases the equipment cannot be brought in, the surface cannot be blasted to the required standard, or the repair area is too small to justify mobilisation.
Cold spray zinc is applied by brush, roller or conventional airless spray. It tolerates a hand-tool cleaned surface better, and it can be applied in a workshop or at height with the same tin. That is its entire commercial case: not that it is better than TSA, but that on a repair it is the only process that will actually be executed.
Cost per square metre, and cost per project
On a large, accessible, blast-cleaned steel surface, TSA is usually cheaper per square metre over the full coating life, because the material is zinc wire and the labour is amortised across a big area. The zinc mass is higher and the coating is a metal, so the protection per dollar is good.
On 20 m² of repairs, TSA is not cheaper at any sensible definition. Mobilisation, blasting containment, compressor, wire, and two skilled operators for a day cost more than the work itself. Cold spray zinc has a higher material cost per kilogram and a much lower fixed cost, which inverts the economics below roughly a few hundred square metres, and inverts it completely for spot repair.
The honest way to compare is to price the whole intervention: access, surface preparation, application, sealing or topcoat, inspection and rework. Quoting material cost per litre against zinc wire per kilogram compares two things that are not sold the same way.
A decision table you can defend in a meeting
| Condition | Thermal spray (TSA) | Cold spray zinc |
|---|---|---|
| New fabrication, large area, blasting available | Preferred | Workable, higher material cost |
| Structure already erected | Rarely practical | Preferred |
| Repair area under 100 m² | Poor economics | Preferred |
| Steel thinner than 4 mm | Distortion risk | Preferred |
| Immersed or buried service | With heavy seal and margin | Not recommended |
| Continuous temperature above 150 °C | Preferred | Binder limit reached |
| Heavy sliding abrasion | Preferred | Topcoat required, still limited |
| C5-M with 25-year durability and access for maintenance | Preferred, sealed | Only as part of a full system |
Two rules of thumb survive most arguments. If the steel can be blasted and the area is large, the metallic coating is the better engineering answer. If the steel cannot be blasted, the area is small, or the work has to be done by two people with a van, the answer is cold spray zinc — and a specification that insists on TSA in that situation usually ends up with no coating at all, or with a coating applied badly.
Questions that come up in every enquiry
Can cold spray zinc replace thermal spray on an offshore C5-M structure?
Not as a standalone coating. At 80 µm it carries roughly a third of the zinc mass of a 120 µm TSA layer, and in the splash zone the abrasion and the chloride load are both high. It can be used as a repair material on a TSA system, and as a zinc-rich primer inside a full C5-M system, but the system and the film build have to be designed together, not substituted one for one.
Which one lasts longer?
On the same structure and in the same environment, a properly sealed TSA system lasts longer, because it holds more zinc. The comparison only becomes interesting when the alternative to cold spray zinc is not TSA but nothing — which is the usual real-world choice on a repair.
Can both be topcoated?
Yes, and both usually should be. TSA needs a sealer at minimum, and a full paint system in C4 and above. Cold spray zinc accepts epoxy and polyurethane topcoats, but the recoat window matters: too early and solvent is trapped, too late and intercoat adhesion drops. Overcoating a cold spray zinc with an epoxy intermediate and a polyurethane finish is the standard way to reach a colour requirement.
Can TSA be applied at low temperature?
The process itself is less temperature-sensitive than a paint, but blasting, sealer and topcoat are not, and most specifications stop work below 5 °C or above 85% relative humidity. Cold spray zinc is subject to the same limits as any solvent-borne coating, so in winter conditions neither process is freely available without a shelter.
What is the right repair for damaged hot-dip galvanizing?
Zinc-rich cold spray at 60–80 µm over a hand-tool cleaned or lightly blasted area, feathered at the edges, is the standard field repair for damaged galvanizing. TSA is technically superior and practically unavailable for a one-metre patch on an erected tower. This is the single most common use of cold spray zinc in our own order book.
Send the structure, get a system
Tell us the component, the steel condition, the environment and the area. The reply is a coating system with film build and consumption — not a price list. If hot-dip is the right answer for your case, we will say so.
More notes
Technical note · 3 November 2026 · 10 min read
Cold galvanizing vs hot-dip: the comparison most articles get wrong
Search "cold galvanizing vs hot-dip galvanizing" and you will find a dozen articles telling you that cold galvanizing gives 5–20 µm of zinc and lasts five to fifteen years. Those articles are not wrong about the process they describe. They are describing electroplating, which is a different thing that happens to share the name. This page compares the material that actually competes with hot-dip galvanizing on structural steel — a zinc-rich cold spray at 80 µm dry film — and shows where the substitution holds and where it does not.
Read the note →