Epoxy primers, intermediate coats and mastics for steel

The role of epoxy materials in the system

Epoxy coatings are the workhorse of most anti-corrosion systems. A two-component binder of resin and curing agent gives a dense film with good adhesion to steel and high resistance to water, many chemicals and abrasion. Within a system, epoxies play several roles:

  • primer — on bare steel or over a zinc-rich primer;
  • intermediate coat — builds up barrier thickness between the primer and the topcoat;
  • mastic — a high-build, surface-tolerant material for maintenance painting and difficult conditions;
  • stand-alone system — indoors, in immersion, on structures where appearance does not matter.

The weak point of epoxies is UV: in sunlight the film chalks and fades, and light colours turn yellow. That is why, outdoors, epoxy coats are overcoated with a polyurethane or polysiloxane topcoat.

Types

High-build and high-solids

In one coat they typically give from 100 to 250–300 µm of dry film, which reduces the number of operations. The higher the volume solids, the less solvent is released during application and curing — important both for the environment and for work in confined spaces.

With micaceous iron oxide (MIO)

The lamellar particles of micaceous iron oxide lie parallel to the surface within the film and lengthen the path of moisture to the metal. Such a film also accepts the next coat better, even after a long interval, so MIO epoxies are often chosen for the intermediate coat when painting at the steel fabrication shop, where members wait a long time before erection.

Surface-tolerant mastics

Designed for surfaces where abrasive blast cleaning is impossible or impractical: maintenance on operating plants, areas with limited access, overcoating sound old coatings. They accept hand and power tool cleaning (St 2 – St 3 to ISO 8501-1), and some products can be applied to damp surfaces. Versions with aluminium flake give a pronounced barrier effect.

Glass flake

Glass flake increases resistance to abrasion and permeation. Such materials are used in areas of mechanical wear and constant wetting, including linings — see “Coatings for tanks and pipelines”.

How to choose an epoxy coating

By its role in the system

The primer, intermediate coat and topcoat are selected together, from one manufacturer’s compatibility list. Each pair of materials has its own minimum and maximum overcoating intervals; once the maximum interval has been exceeded, the surface is abraded before the next coat.

By surface preparation

For new construction where abrasive blast cleaning is possible, primers and intermediate coats for Sa 2½ are chosen. For maintenance with St 2 – St 3 preparation, use mastics whose technical data sheets explicitly permit such preparation. A mastic over rust protects less well than the same mastic over a blast-cleaned surface, so the service life of maintenance painting is estimated with the preparation in mind.

By application conditions

Epoxies have a minimum curing temperature. For winter work, manufacturers offer low-temperature curing agents and winter grades, and some products cure at 0 °C and below. In summer heat, by contrast, the pot life becomes shorter: mix only as much as can be applied in time and do not leave mixed paint in the sun.

By service environment

For immersion, chemical contact and service under insulation, choose products whose technical data sheets explicitly list these conditions. Resistance to the specific medium is confirmed by the questionnaire: substance, concentration, temperature and regime.

Common mistakes

  • An epoxy used as the exterior topcoat where appearance matters.
  • Wrong mixing ratio: mixing part of a kit “by eye”, insufficient stirring.
  • Application to cold steel below the permitted temperature, or when the surface is not at least 3 °C above the dew point.
  • Overcoating after the maximum interval without abrading — delamination of the next coat.
  • Excessive thickness in a single pass with solvent-borne materials — solvent retention, blistering, slow cure.
  • Rain or dew on a fresh coat: the surface turns white and has to be sanded before the next coat.
  • A winter curing agent used in summer — the pot life runs out far too quickly.

See also: corrosion protection — overview, zinc-rich primers, topcoats.

Materials from leading manufacturers

  • Interseal 670HS — International (AkzoNobel) — surface-tolerant high-build epoxy for atmospheric exposure and immersion.
  • Intergard 475HS — International (AkzoNobel) — intermediate coat, also available with micaceous iron oxide.
  • Interzone 954 — International (AkzoNobel) — high-build modified epoxy for the splash zone; continues to cure under water.
  • Jotamastic 90 — Jotun — epoxy mastic for maintenance painting; a winter grade is available.
  • Penguard Express — Jotun — fast-curing high-build primer for new construction.
  • Macropoxy EG-1 Plus — Sherwin-Williams — primer and intermediate coat with micaceous iron oxide.
  • Carbomastic 15 — Carboline — aluminium mastic for steel with minimal surface preparation.
  • Carboguard 890 — Carboline — chemical-resistant self-priming mastic.
  • PPG AMERLOCK 2 C / SIGMACOVER 2 — PPG — self-priming epoxy that cures down to 0 °C.
  • EPOTECT HB — Kansai Paint — high-build epoxy paint for the intermediate coat over zinc primers.
  • EPOTECT MIO — Kansai Paint — epoxy paint with micaceous iron oxide (MIO) and a polyamide curing agent.
  • Penguard Primer — Jotun — polyamide-cured epoxy primer for atmospheric and immersed service.
  • Penguard Express ZP — Jotun — fast-curing high-build epoxy primer with zinc phosphate.
  • Penguard Express MIO — Jotun — fast-drying high-build epoxy coating with micaceous iron oxide (MIO).
  • TEKNOMASTIC 80 PRIMER — Teknos — epoxy primer with 82 % volume solids, compliant with NORSOK M-501 (systems 1, 7A, 7B); maintenance painting over St 2 preparation; winter curing agent for use down to −5 °C.
  • TEKNOMASTIC COMBI 80 E — Teknos — high-solids epoxy certified to ISO 12944-9: CX for areas subject to impact and Im4 immersion.
  • TEKNOPLAST HS 150 — Teknos — high-build epoxy paint — primer and topcoat for buried and underwater structures; also a vapour barrier on concrete.
  • TEKNOPLAST PRIMER 7 — Teknos — epoxy primer with 70 % volume solids for steel, zinc and aluminium; overcoating interval of up to 12 months.
  • TEKNOPLAST PRIMER 7 MIOX — Teknos — epoxy primer and intermediate coat with micaceous iron oxide (MIOX), compliant with SSG 1021-GS.
  • INERTA 165 GF — Teknos — glass-flake epoxy: 500–1000 µm per coat direct to metal, NORSOK M-501 system 7A.
  • Macropoxy 646 — Sherwin-Williams — 1:1 polyamide epoxy primer-finish for atmospheric and fresh or salt water immersion service.
  • Dura-Plate 235 — Sherwin-Williams — self-priming multi-purpose epoxy for water immersion, applicable down to −18 °C.
  • PPG SIGMACOVER 456 — PPG — high-build recoatable epoxy buildcoat or finish, cures down to −10 °C.
  • PPG SIGMACOVER 280 — PPG — aluminium-pigmented universal epoxy primer for steel, galvanising and non-ferrous metals.
  • PPG SIGMAFAST 278 — PPG — fast-curing high-solids zinc phosphate epoxy primer and buildcoat, cures down to −5 °C.
  • Intergard 251 — International (AkzoNobel) — fast-drying zinc phosphate epoxy primer for shop priming and chemical environments.
  • Intergard 269 — International (AkzoNobel) — quick-drying epoxy blast holding primer and tie coat over zinc silicate.
  • Intergard 345 — International (AkzoNobel) — fast-curing high-solids zinc phosphate epoxy primer-finish, 100–150 µm per coat.
  • Intershield 300 — International (AkzoNobel) — abrasion-resistant aluminium pure epoxy for platforms, decks and underwater hulls.
  • Jotacote Universal N10 — Jotun — universal pure epoxy primer for atmospheric and immersed service, cures down to −10 °C.
  • Penguard Pro — Jotun — universal surface-tolerant epoxy for primer, midcoat or finish, 100–500 µm per coat.
  • Jotamastic 87 — Jotun — surface-tolerant epoxy mastic for maintenance, winter grade applicable down to −5 °C.
  • ESCO — Kansai Paint — moisture-curing epoxy primer that penetrates rusty steel, for maintenance over St 3.
  • EPOTECT GF(M) — Kansai Paint — high-build glass flake epoxy, 250 µm per coat, resistant to fresh and sea water.
  • PPG SIGMAPRIME 200 — PPG — universal pure epoxy primer for ballast and cargo tanks, decks and hulls.
  • Macropoxy 400 — Sherwin-Williams — multi-functional zinc phosphate epoxy primer, build coat and low-sheen finish for C1–C5.
  • Macropoxy M922 — Sherwin-Williams — high-solids glass flake epoxy, up to 1000 µm per coat, applicable to hot steel.
  • PPG SIGMACOVER 410 — PPG — high-build polyamide epoxy intermediate coat curing down to −5 °C, LT version available.
  • PPG SIGMACOVER 350 — PPG — surface-tolerant pure epoxy primer-coating, compatible with various aged coatings.
  • Interseal 1052 — International (AkzoNobel) — fast-curing high-solids anti-corrosive epoxy, 75–250 µm per coat, zinc phosphate or MIO.
  • Interzone 505 — International (AkzoNobel) — high-build glass flake epoxy, 300–500 µm per coat, for atmospheric and immersion service.
  • Intergard 740 — International (AkzoNobel) — high-gloss two-pack epoxy finish for primed steel and concrete, resists chemical spills.
  • Jotamastic 80 — Jotun — surface-tolerant epoxy mastic for maintenance over aged coatings, applicable below 0 °C.
  • Penguard Universal — Jotun — universal pure epoxy for atmospheric and immersed service, applicable from −10 °C.
  • Penguard HB — Jotun — high-build all-purpose polyamide epoxy for new construction, atmospheric and immersed service.
  • PPG AMERLOCK 400 C / SIGMACOVER 400 — PPG — surface-tolerant universal epoxy for St 2 steel, damp surfaces and aged coatings.
  • Interplus 356 — International (AkzoNobel) — surface-tolerant aluminium MIO epoxy primer, cures down to −5 °C.
  • Penguard HB II — Jotun — matt high-build polyamide epoxy, applicable at sub-zero surface temperatures down to −10 °C.
  • INERTA MASTIC — Teknos — low-solvent epoxy mastic for maintenance over St 2 steel, winter hardener to −5 °C.
  • Intercure 200HS — International (AkzoNobel) — zinc phosphate MIO epoxy primer, rapid recoat, cures at low temperatures.
  • Interzone 1000 — International (AkzoNobel) — glass flake epoxy for splash zones, jetties and subsea steel, 500–1000 µm.
  • Jotamastic Plus — Jotun — surface-tolerant epoxy mastic for maintenance over St 2 steel and aged coatings.
  • Macropoxy C401 — Sherwin-Williams — micaceous iron oxide epoxy intermediate over Macropoxy and Zinc Clad primers.
  • Macropoxy M905 — Sherwin-Williams — low-temperature curing epoxy intermediate down to 0 °C, rapid overcoating in cold weather.
  • Dura-Plate 301W — Sherwin-Williams — surface- and humidity-tolerant epoxy primer for wet, flash-rusted steel, no dew-point limits.
  • Jotamastic SF — Jotun — solvent-free surface-tolerant epoxy mastic for maintenance over St 2, winter grade available.
  • Macropoxy 4600 — Sherwin-Williams — high-solids zinc phosphate epoxy primer for steel in C1–C5 atmospheres, applicable from −5 °C.

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 an epoxy be left without a topcoat?

Indoors and where appearance does not matter, often yes, if the manufacturer’s system allows it. Outdoors, epoxy chalks and fades, so a topcoat is needed to keep the appearance and to protect the epoxy coats from UV.

How does a mastic differ from an ordinary epoxy paint?

A mastic is a high-solids, high-build material with greater tolerance of surface preparation: it can be applied over hand- and power-tool-cleaned surfaces and over sound old coatings. Ordinary epoxy primers and intermediate coats are designed for a higher standard of preparation.

What does micaceous iron oxide do?

The lamellar pigment strengthens the barrier and improves adhesion of the next coat, especially when a long time passes between operations.

Can epoxies be applied in winter?

Yes, if a material or curing agent for low temperatures is chosen and the dew point condition is met. In the cold, the time before overcoating increases several-fold, and this must be allowed for in the programme.

We will select epoxy primers, intermediate coats or a repair mastic for your project — call us or message us on WhatsApp/Telegram: +998 94 094 52 27.