Zinc-rich primers: zinc silicate and epoxy zinc

How a zinc-rich primer works

Most of the dry film of such a primer consists of fine zinc dust. The particles are in contact with one another and with the steel, so the coating conducts current and acts as a sacrificial anode: zinc is electrochemically more active than iron and, in the presence of moisture, corrodes first, protecting the steel in pores and at small areas of damage. The zinc corrosion products gradually fill the pores and seal the film. Hence the main condition: close contact with clean metal — over mill scale, rust or dust the sacrificial effect is lost.

Zinc-rich primers are the basis of systems for high corrosivity categories and long service lives. ISO 12944-5 treats them as a separate group of primers (designation Zn (R)) with a requirement for a minimum zinc dust content in the dry film. In American practice their composition is governed by SSPC Paint 20: inorganic primers are Type I, organic primers are Type II, and by zinc content the standard divides them into levels, Level 1 being the highest. These designations often appear in technical data sheets.

Two types: zinc silicate and epoxy zinc

Parameter	Inorganic (zinc silicate)	Organic (epoxy zinc)
Binder	ethyl silicate (alkyl silicate); final cure by moisture in the air	epoxy resin with a curing agent
Steel preparation	abrasive blast cleaning, usually Sa 2½ with a specified profile	usually Sa 2½; some products accept power tool cleaning — see the technical data sheet
Heat resistance	high: for the materials in this guide, from 400 °C continuous to 540 °C short-term or under a topcoat	limited by the epoxy binder — according to manufacturers, around 120–150 °C continuous
Curing conditions	needs humidity in the air: in dry weather curing slows down	needs a temperature above the lower limit and a dry surface above the dew point
Overcoating	porous film: full cure and a “mist” first pass of the next coat are required	easier to overcoat with epoxies and polyurethanes
Sensitivity to thickness	high: excessive thickness causes mud-cracking	moderate, within the maximum coat thickness
Typical uses	new construction, shop painting, hot surfaces, slip-critical connections	new construction and maintenance, touch-up of zinc silicate and damaged galvanising

How to choose a zinc-rich primer

Environment and service life

Zinc primers are particularly important in systems for categories C4, C5 and CX to ISO 12944-2 and for high durability. In milder environments they are also used where a long service life is needed or where the design specifically calls for a primer with sacrificial action.

Where the painting will be done

Zinc silicate is easier to apply in the shop, where abrasive blast cleaning, the right air humidity and thickness control are easier to achieve. On site, for maintenance and touch-up, epoxy zinc primers are usually chosen — they are less demanding about conditions and easier to overcoat.

Service temperature

Hot surfaces require zinc silicate and a compatible heat-resistant topcoat — see the “Heat-resistant coatings” page for details. Epoxy zinc primers are limited by the service temperature of the epoxy binder.

Slip-critical connections

If the structure is assembled with preloaded high-strength bolts, the primer on the faying surfaces must have a proven slip coefficient. In American practice this is a class under RCSC (for example, Class B); in European practice, a slip factor test to EN 1090-2. Such data are given in the technical data sheets of several primers in this section.

Substrate

Zinc-rich primers are intended for carbon steel. They are not applied to stainless steel — manufacturers offer separate systems for it. Hot-dip galvanised steel has its own preparation requirements, and epoxy zinc primers are intended for repairing damaged galvanising.

Application conditions in Uzbekistan’s climate

Dry air. Ethyl silicate zinc primers develop their properties through moisture in the air. Manufacturers specify a minimum humidity during curing — around 50–60 % for a number of products — and offer ways to speed up the process: water misting and special accelerators. In a dry summer this has to be built into the method statement and the programme; overcoating an under-cured coat leads to defects in the whole system.
Heat. On hot steel the solvent evaporates while still in the spray fan, causing dry spray and porosity. Slow thinners specified in the technical data sheet, working in the morning and shading all help.
Cold and dew. The surface must be dry and, as technical data sheets require, at least 3 °C above the dew point. The minimum application temperature varies between products: for some it is below zero, for others +5 °C.
Dust. Dust on the cleaned surface impairs the contact between the zinc and the steel; the amount of dust after cleaning is checked, for example, to ISO 8502-3.

Overcoating zinc silicate primer

The film of an inorganic zinc primer is porous. If a thick coat of epoxy or polyurethane is applied straight over it, air escapes from the pores through the fresh paint and leaves bubbles and craters. Manufacturers therefore require, first, that the primer has cured — the degree of cure is checked, for example, by a solvent rub test to ASTM D4752, with the method and acceptance criterion given in the technical data sheet. Second, the first pass of the next material is applied as a thin “mist” coat, followed by the full coat. Before overcoating, zinc salts and contamination are removed from the surface.

Common mistakes

Excessive zinc silicate thickness, especially in corners and on edges: mud-cracking appears and the coat has to be removed.
Application over an inadequately cleaned surface — over mill scale, dust or salts.
Overcoating uncured zinc silicate in dry weather.
Skipping the “mist” coat when overcoating — bubbles and craters in the finish system.
Alkyd and oil-based paints over a zinc primer: several manufacturers state explicitly that such materials are not suitable for overcoating.
Epoxy zinc primer on surfaces heated above its permitted temperature.
Leaving a zinc primer without a following coat for a long time in an aggressive atmosphere: zinc salts build up on the surface and must be removed before the next coat.

Materials from leading manufacturers

Inorganic (zinc silicate)

Interzinc 22 — International (AkzoNobel) — zinc silicate primer with rapid overcoating technology.
Resist 86 — Jotun — zinc ethyl silicate for highly corrosive environments, heat-resistant up to 540 °C, RCSC Class B.
Zinc Clad ZS — Sherwin-Williams — single-component ethyl silicate zinc primer; at low film thickness, a weldable shop primer.
Carbozinc 11 — Carboline — inorganic zinc primer that cures at temperatures down to −18 °C.
PPG SIGMAZINC 158 — PPG — two-component zinc ethyl silicate for substrate temperatures from −90 to 500 °C.
ZINCTECT 1500 — Kansai Paint — high-build zinc silicate, about 75 µm per coat.
ZINCTECT 1000 — Kansai Paint — fast-drying zinc silicate shop primer for plate and sections.
Resist 78 — Jotun — inorganic zinc ethyl silicate primer for highly corrosive environments, heat resistant up to 540 °C.
TEKNOZINC ESI 3180 — Teknos — fast-drying, moisture-curing zinc ethyl silicate: dry heat up to +400 °C, application down to −5 °C.
TEKNOZINC SS — Teknos — two-component zinc ethyl silicate for atmospheric exposure and immersion in solvents; also a primer for heat-resistant systems up to +400 °C.
Zinc Clad II Plus — Sherwin-Williams — three-component inorganic zinc ethyl silicate primer with 83% zinc, Class B slip.
PPG DIMETCOTE 9 — PPG — moisture-curing inorganic zinc ethyl silicate primer, Class B slip, up to 500 °C.
Jotatemp 540 Zinc — Jotun — glass flake reinforced inorganic zinc silicate primer for hot equipment up to 540 °C.
Resist ZES — Jotun — moisture-curing inorganic zinc ethyl silicate primer for steel, 400 °C continuous, 540 °C peak.

Organic (epoxy zinc)

Interzinc 52 — International (AkzoNobel) — epoxy zinc primer with 80 % zinc.
Jotun Barrier — Jotun — epoxy zinc primer for environments up to CX; repair of zinc silicate and galvanising.
Zinc Clad R Plus — Sherwin-Williams — epoxy zinc primer for systems with Macropoxy, Dura-Plate and Acrolon.
Carbozinc 859 — Carboline — epoxy zinc primer with rapid overcoating and Class B for slip-critical connections.
ZINCTECT 500 — Kansai Paint — high-build epoxy zinc primer with a polyamide curing agent.
Jotun Barrier 80 — Jotun — epoxy zinc-rich primer for atmospheres up to C5 and for repairing zinc silicate coatings and galvanising.
TEKNOZINC 90 SE — Teknos — epoxy zinc primer with at least 90 % zinc in the dry film and an overcoating interval of up to 18 months.
TEKNOZINC 80 SE — Teknos — epoxy zinc primer with 80 % zinc — the standard primer in Teknos systems for C4, C5 and immersion.
TEKNOZINC 3231 — Teknos — epoxy zinc primer with a high zinc content (at least 90 %) for use under polysiloxane and polyurethane topcoats.
PPG SIGMAZINC 102 HS — PPG — high-solids zinc-rich epoxy primer, 60–150 µm per coat, overcoatable after 6 hours.
Interzinc 315 — International (AkzoNobel) — rapid-recoat epoxy zinc-rich primer, cures below 0 °C, Class B slip.
Jotun Barrier 77 — Jotun — zinc-rich epoxy primer for atmospheres up to C5 and for repairing galvanising.
Zinc Clad IV EU — Sherwin-Williams — two-pack epoxy zinc-rich primer for blast-cleaned steel and galvanizing repair.
Jotun Barrier 90 — Jotun — epoxy zinc-rich primer with very high zinc dust content, up to C5.
Zinc Clad 4700 — Sherwin-Williams — two-pack epoxy zinc primer for blast-cleaned steel to SSPC Paint 20.
Zinc Clad IV 85% — Sherwin-Williams — epoxy zinc-rich primer with 85% zinc, Class A slip for friction joints.
Interzinc 72 — International (AkzoNobel) — epoxy zinc-rich primer with 90% zinc for steel in aggressive environments.
TEKNOZINC 3233 — Teknos — one-pack moisture-cure zinc-rich urethane primer for steel structures.
PPG AMERCOAT 68 HS — PPG — three-component zinc-rich epoxy primer, over 80 % zinc in film, low-temperature accelerator.
PPG SIGMAZINC 109 HS — PPG — high-solids zinc-rich epoxy primer, recoat after 4 h, application down to 0 °C.
Zinc Clad J984BS — Sherwin-Williams — epoxy zinc-rich primer to BS 4652, fast drying, repair of damaged galvanizing.
Jotun Barrier ZEP — Jotun — polyamide zinc epoxy primer for structural steel in environments up to C4.

This list is not exhaustive. The final choice is made on the basis of the manufacturer’s documents and the project requirements.

Frequently asked questions

Can a zinc-rich primer be replaced with an epoxy primer?

Technically, yes: there are zinc-free systems for the same corrosivity categories. But they have no sacrificial action, and the system is usually thicker. If a zinc primer is specified in the design, any substitution must be agreed with the designer.

Is a zinc primer needed over hot-dip galvanising?

No — galvanising is itself a zinc coating. To paint over it, primers approved by the manufacturer for galvanised steel are used after appropriate preparation. Epoxy zinc primers are used to repair damaged areas of galvanising.

Why does zinc silicate crack?

Most often because of excessive thickness, especially in corners and on edges, where paint accumulates. The maximum coat thickness is given in the technical data sheet and is checked during application.

How long should I wait before overcoating zinc silicate?

It depends on the product, the temperature and the humidity: the technical data sheet gives minimum intervals for different conditions. In dry weather, go by the actual degree of cure, not just by the number of hours.

One-pack cold galvanising compounds — for stand-alone protection of steel and repair of galvanising and welds — are collected in the section Cold galvanising.

We will select a zinc-rich primer and the complete system for your project — call us or message us on WhatsApp/Telegram: +998 94 094 52 27.