Cold galvanising: zinc-rich compounds and galvanising repair

What is meant by cold galvanising

Cold galvanising is the protection of steel with a compound whose dry film consists mostly of zinc powder — from 90 to 96 % for the materials in this section. The compound is applied by brush, roller, spray or aerosol at ambient temperature, without dipping the article into molten zinc. The zinc particles touch one another and the metal, so the film conducts current and corrodes instead of the steel, and the steel does not rust even where the coating has been scratched. Some manufacturers call such materials film galvanising or galvanising repair compounds.

In terms of standards, this is a paint coating rather than a metallic one: ISO 14713-1 lists the processes for applying zinc coatings — hot-dip galvanising, sherardising, thermal spraying, mechanical plating and electroplating — and zinc paints are not on that list. Cold galvanising is therefore selected and tested as part of a paint system under the ISO 12944 series, and its properties are defined by the technical data sheet of the specific product.

How it differs from hot-dip galvanising and zinc-rich primers

Parameter	Hot-dip galvanising	Cold galvanising	Two-component zinc-rich primers
How the coating is produced	the article is dipped into a bath of molten zinc at the plant	a ready-to-use compound is applied by brush, roller, spray or aerosol	the base is mixed with a curing agent and sprayed as the first coat of the system
Bond to steel	metallurgical: layers of zinc–iron alloy form at the interface	adhesion of the film and electrical contact of the zinc particles with the metal	as for cold galvanising, but the binder cures chemically
Where it is done	only at the plant; the size of the article is limited by the bath	in the shop and on site, including on erected structures	mainly in the shop; epoxy zinc primers also on site
Components	—	usually one component, ready to use	two or three components; the mix has a limited pot life
Role in the system	stand-alone coating or the base of a duplex system	stand-alone protection, repair coat or primer under a topcoat	a primer that is almost always overcoated with intermediate and finish coats
Renewal in service	re-galvanising is only possible after dismantling	a new coat is applied over the cleaned old one without removing it	damage is repaired with the manufacturer’s repair system

Where cold galvanising is used

Repair of hot-dip galvanising. Welding, flame cutting, drilling on site and impacts during transport and erection leave areas on galvanised steel where the zinc has burnt off or been knocked off. ASTM A780 describes three types of repair material: zinc-based solders, paints containing zinc dust and sprayed zinc. Whether such repair is permitted for areas left uncoated at the galvanising plant is determined by the standard for the article or the coating itself.
Welded joints and bolted connections. When galvanised structures are erected, welds, cut edges and connections are covered with a zinc compound after assembly so that the protection remains continuous.
Stand-alone protection. For structures that do not fit into the bath, are already erected or cannot be taken to the plant. Some compounds have been tested by their manufacturers as stand-alone systems to ISO 12944-6 and ISO 12944-9 — check the thickness at which the result was obtained.
Renewal of old galvanising and metallising. When the zinc layer has become thin, it is “recharged” with a new coat without complete removal, after the corrosion products have been cleaned off.
Duplex systems. The zinc layer is overcoated with compatible intermediate and finish materials — for colour, resistance to aggressive environments and a longer service life.

How to choose a compound

Zinc content and purity

Compare the zinc content in the dry film and by mass: a figure for the liquid material or by volume looks different. Data sheets confirm the purity of the zinc by reference to a standard for zinc dust, for example ASTM D520. A paint with noticeably less zinc, or with no zinc content stated, may work mainly as a barrier — it should not be regarded as cold galvanising.

Binder and curing mechanism

Many one-component compounds dry physically, by solvent evaporation, and the next coat partly redissolves the previous one — which is convenient for repairs. There are also compounds with an epoxy ester binder that become touch dry quickly, and water-based zinc silicate systems with high heat resistance that need air humidity to cure. The binder determines the thinner, the overcoating intervals and the choice of topcoat.

Compatibility with topcoats

Under an alkyd paint, zinc forms soaps and the coating peels, so most manufacturers allow only non-saponifiable materials over a zinc layer — epoxies, polyurethanes, acrylics. Some compounds are not intended to be overcoated at all, while others require a “mist” first coat. The final word belongs to the data sheet of the specific product and a trial patch.

Service conditions

Take into account the corrosivity category to ISO 12944-2, the required durability, the temperature and contact with water or process media. Heat resistance varies widely between data sheets: the one-component organic compounds in this section are designed for continuous service up to +115…+177 °C, some products withstand dry heat up to +300 °C, and the water-based zinc silicate up to +450 °C continuously. Data sheets also give the permissible pH range separately for atmospheric exposure and for immersion; resistance to a specific medium is confirmed using our questionnaire.

Surface preparation and application

Bare metal. Protection arises only when the zinc is in direct contact with the steel, so oil and grease are removed and rust, mill scale and old paint are taken off completely. The best result is given by abrasive blast cleaning to Sa 2½ to ISO 8501-1; for repairs many data sheets accept power tool cleaning.
Galvanised surfaces. Before repair, salts are washed off and white rust is removed, and smooth galvanising is roughened — by light abrasive sweeping or mechanically. Preparation of hot-dip galvanised steel for painting is described in ASTM D6386.
Welds and edges. Slag and spatter are removed and sharp edges are rounded; preparation grades for welds and edges are set out in ISO 8501-3.
Stirring. Heavy zinc settles quickly: the compound is stirred mechanically until uniform, and this is repeated during work.
Stripe coating. Edges, bolts, nuts and welds are first painted by brush, then the full coat is applied.
Thickness control. The wet film is checked with a comb gauge during work, and the dry film with a gauge, allowing for surface roughness, to ISO 19840.
Climate. In hot weather, watch the maximum surface temperature — from +30 to +60 °C depending on the product — and with morning dew, wait until the steel is warmer than the dew point by the margin stated in the data sheet.

Common mistakes

Buying a “zinc” paint without data on the zinc content of the dry film.
Application over rust, old paint or after a rust converter: the insulating film breaks the contact between the zinc and the steel.
Repairing galvanising without removing white rust and salts.
An alkyd or nitrocellulose topcoat over the zinc layer without approval in the data sheet.
A thick topcoat over fresh zinc without a “mist” coat — bubbles and craters.
Thinning with a random solvent when the data sheet requires the manufacturer’s own thinner.
Aerosols on large areas: the film comes out thin and many cans are used.

What the standards say

ISO 1461:2022 — requirements for hot-dip galvanised coatings on fabricated articles; after-treatment and additional coatings of galvanised articles are outside its scope.
ISO 14713-1:2017 — guidance on zinc coatings; for maintaining the protection of galvanised steel in service it refers to ISO 12944-5 and ISO 12944-8.
ISO 12944-5:2019, ISO 12944-6:2018 and ISO 12944-9:2018 — types of paint systems, laboratory performance testing and requirements for systems for offshore conditions of categories CX and Im4.
ASTM A780/A780M-20 — repair of damaged and uncoated areas of hot-dip galvanised coatings, including with paints containing zinc dust.
ISO 3549:2024 and ASTM D520 — requirements for zinc dust pigments for paints.
HG/T 4845-2015 — the Chinese industry standard for cold galvanising paints; useful when materials are supplied from China.

Corrosion protection codes of the CIS countries and Uzbekistan are covered in the “Anti-corrosion standards” section.

Materials from leading manufacturers

ZINGA — ZINGA (Zingametall) — one-pack coating with 96 % zinc: stand-alone protection, duplex primer and galvanising repair.
ZINGA 420 — ZINGA (Zingametall) — low-VOC version of ZINGA: 96 % zinc, 110 µm system tested to C5.
ZINGALU — ZINGA (Zingametall) — zinc-aluminium coating with a galvanised look for repair and touch-up of galvanised steel.
ZINGASPRAY — ZINGA (Zingametall) — aerosol ZINGA with 96 % zinc for touch-up of welds, edges and galvanising.
AQUAZINGA — ZINGA (Zingametall) — two-pack water-based zinc silicate with 94 % zinc, solvent-free, up to 450 °C.
ZRC Cold Galvanizing Compound — ZRC Worldwide — one-pack compound with 95 % zinc for galvanising repair to ASTM A780.
ZRC Galvilite — ZRC Worldwide — silvery 95 % zinc compound matching new galvanising, for galvanising repair.
ZRC 221 — ZRC Worldwide — cold galvanizing compound with 92 % zinc and VOC reduced to 221 g/l.
Fosroc Galvafroid — Fosroc — zinc-rich cold galvanising paint with 90 % zinc for brush and roller.
Rust-Oleum 1085 Cold Galvanising — Rust-Oleum — fast-drying epoxy ester primer with 90 % zinc for galvanising and weld touch-up.

The list includes compounds that their manufacturers classify as cold or film galvanising. Two-component epoxy zinc and zinc silicate materials for new-build systems are collected on the zinc-rich primers page.

Selection and calculation for your project

For a first estimate of the environment category and system type, there is an online corrosion protection system selector. We select and calculate the compound, thickness and quantity for a specific task — repair of galvanising after erection, protection of welded joints, a stand-alone system or a duplex system — on request as part of our protective coatings engineering. For the calculation we need a description of the structure and its coating, the repair area, the service conditions and the schedule.

Frequently asked questions

Can cold galvanising replace hot-dip galvanising?

Sometimes, but this is a design decision, not a default substitution. Hot-dip galvanising gives a factory coating with an alloy bond; cold galvanising gives a paint film whose properties depend on preparation, thickness and application conditions. If the design specifies hot-dip galvanising, any substitution is agreed with the designer.

Does cold galvanising need a topcoat?

Not always. Without a topcoat the layer is easier to renew: new zinc is applied directly over the cleaned old coat. In an aggressive environment, where colour matters or where maintenance is impossible, a duplex system is chosen from materials whose compatibility the manufacturer confirms.

How do you restore galvanising after welding on site?

The weld is cleaned of slag, spatter and burnt zinc down to bright metal, taking in the edges of the sound coating; white rust is then removed and the zinc compound is applied as a brush stripe coat and a full coat to the design thickness, which is checked with a thickness gauge.

Aerosol or tin — which should you choose?

An aerosol is convenient for touching up welds, edges and small areas of damage. Structures and large areas are coated with a compound from a tin — by brush, roller or spray: this makes it easier to build up and check the thickness.

We will select a cold galvanising compound and calculate the quantity for your project — call us or message us on WhatsApp/Telegram: +998 94 094 52 27.