ISO 12944 paint systems: example schemes for C3, C4, C5 and immersion

An ISO 12944 paint system is chosen by crossing the corrosivity category of the environment (C3, C4, C5 or immersion) with the target durability (L, M, H or VH). The tables in the annex of ISO 12944-5 then give, for each combination, the primer type, the allowed binders, the number of coats and the total nominal thickness. The technical data sheet of the selected product sets the application values.

This guide is for maintenance managers, buyers and project managers who write or review a paint specification for steel in Quebec or Ontario. It does not reproduce the tables of the standard: they are copyrighted, and a thickness taken out of context often leads to the wrong system. It explains how those tables are built, how to read a typical scheme and which questions to ask before approving a system. For the basics of the standard, see our article ISO 12944 explained.

A system is defined by two inputs: corrosivity and durability

The starting point of an ISO 12944 system is always the pairing of corrosivity and durability, never the product. Until both inputs are fixed, no table can be read correctly.

The corrosivity category comes from ISO 12944-2. For atmospheric exposure it runs from C1 to CX; the common industrial cases are:

• C3 (medium): urban or moderate industrial atmosphere, interiors of production buildings with some humidity and pollution.
• C4 (high): industrial areas, moderate coastal areas, chemical plants, buildings with frequent condensation.
• C5 (very high): aggressive industrial atmosphere, near-permanent humidity, exposed coastal areas.

For immersion and burial, the same part defines Im1 (fresh water), Im2 (sea or brackish water) and Im3 (soil). ISO 12944-9 covers the extreme cases (CX and Im4) specific to offshore structures.

Durability comes from ISO 12944-1. It is expressed in four classes: L (low), M (medium), H (high) and VH (very high). Each class corresponds to a range of years defined in the standard until the first major maintenance repainting. It is not a guarantee: it is a technical notion used to select the system and plan maintenance.

The logic of a scheme: primer, intermediate, topcoat

Each coat in a system has a specific role, and that role determines the binder. An ISO 12944-5 scheme is read from the bottom up.

• The primer provides adhesion to the prepared steel and, depending on its type, active protection. A zinc-rich primer protects steel galvanically; a zinc-free epoxy primer protects by barrier effect. See our entry on zinc-rich primer.
• The intermediate coat or coats build thickness and barrier effect. These are often high-solids epoxies, sometimes loaded with lamellar pigments.
• The topcoat resists UV, weathering and sometimes chemicals, and provides the colour. Aliphatic polyurethanes are common outdoors, because epoxy chalks in sunlight.

In immersion the logic changes: there is no decorative topcoat, and the system often consists of several coats of the same epoxy binder, or of a novolac epoxy when temperature or chemistry requires it. Our entry on novolac epoxy explains its use.

Binders allowed in the ISO 12944-5 tables

The ISO 12944-5 tables group systems by binder family, and the binder choice matters more than the product name. The standard uses generic codes: for example EP (epoxy), PUR (polyurethane), ESI (ethyl silicate), AK (alkyd), AY (acrylic), and the Zn (R) designation for a zinc-rich primer.

This table summarizes common roles; it does not replace reading the tables of the standard, which specify the allowed combinations for each system.

Total nominal thickness: where to find it and how to check it

The total nominal thickness of a system is read in the ISO 12944-5 table that matches the selected corrosivity and durability, and nowhere else. It increases with corrosivity and with durability, and it is then split between coats according to each product's data sheet.

Three rules prevent most errors:

• Thickness is dry and nominal. It is the target dry film thickness, measured after cure. See our entry on dry film thickness.
• Inspection follows a measurement standard. ISO 19840 sets the measurement method and acceptance criteria on rough surfaces; some North American specifications use SSPC-PA 2 instead.
• The data sheet sets the limits per coat. It gives the minimum and maximum thickness per coat, the recoat window and the application conditions. Exceeding the maximum can cause mud cracking or incomplete cure.

Example schemes for C3, C4, C5 and immersion

The examples below describe the logic of the systems found in the tables, without thicknesses: the exact values are read in ISO 12944-5 and in the manufacturer's data sheet.

In immersion, film continuity matters as much as thickness: a holiday test finds discontinuities before start-up. When cathodic protection is planned, the coating must be compatible with it, as ISO 12944-9 and the data sheet specify.

What the standard does not decide for you

ISO 12944-5 gives generic systems; it does not choose the product, does not account for every site constraint and does not replace testing. Four points remain to be decided in the specification:

• The surface preparation actually achievable. ISO 12944-4 and ISO 8501-1 set the grades; if abrasive blasting is impossible, the system must be revisited. See our NACE No. 2 / SSPC-SP 10 guide.
• Application conditions. Temperature, humidity and dew point must meet the data sheet; ISO 12944-7 covers execution and supervision.
• Performance tests. ISO 12944-6 defines the laboratory tests used to qualify a system for a given corrosivity and durability.
• The inspection plan. ISO 12944-8 helps write the full specification, including reference areas and hold points.

How to validate a system before work starts

A system is valid when corrosivity, durability, preparation, product and inspection tell the same story. Before signing, check that:

• the corrosivity category is justified by the actual environment, not by habit;
• the durability class matches the maintenance cycle planned by the operator;
• the selected system appears in the ISO 12944-5 tables or has passed the ISO 12944-6 tests;
• each product has its data sheet, with thickness per coat and recoat windows;
• the inspection plan covers thickness measurement, adhesion and, in immersion, holiday testing.

Our crews and qualified subcontractors apply these systems from LaSalle, in Montreal, throughout Quebec and Ontario, for example in Montreal, in Montérégie, on the Côte-Nord and in Ontario. The service is described on the industrial painting page, and the full range of systems on the protective coatings page. Send the specification, photos and dimensions through the request a quote form for a quote within 24 business hours, pricing on consultation.

Frequently asked questions

What paint thickness is needed in C4 or C5?

The total nominal thickness is read in the ISO 12944-5 table that matches the target corrosivity and durability, then split between coats according to the data sheet. A number given without those two inputs has no value.

Is the VH durability class a guarantee?

No. ISO 12944-1 states that durability is a technical notion that helps select a system and plan maintenance. A guarantee, if any, is a contractual matter.

Can a C3 system be used in a C4 environment by adding thickness?

Not simply. Moving from C3 to C4 often changes the primer type and the allowed binders, not just the thickness. Start again from the table for C4.

Is a topcoat needed in immersion?

Usually not. In immersion, the system is designed for chemical resistance and barrier effect; a polyurethane topcoat adds nothing under water. Our article on epoxy coating service life details these systems.