Anatomy of a Coating System
What a coating is made of, and how primers, stripe coats, intermediate coats and topcoats work together as a system to protect steel and concrete.
Key takeaways
- Every liquid coating combines four ingredient groups: a binder (film former), pigments and extenders, a carrier (solvent or water) and additives.
- Protective systems are built in layers, each with a job: the primer bonds and fights corrosion, intermediate coats add barrier thickness, and the topcoat resists weather and service.
- Performance comes from the whole system — surface preparation, compatible products, correct film thickness per coat and proper recoat timing.
- ISO 12944-5 gives example systems by corrosivity category and durability; the manufacturer’s data sheets govern the specifics.
A “coating” on a bridge, tank or factory floor is rarely one product. It is a system: a prepared surface plus two or more layers, each formulated for a different task. Understanding both levels — what goes into a single can, and how the layers cooperate — makes it far easier to read a specification, compare bids and diagnose failures.
What’s in a can of coating
Binder
The binder (resin) is the continuous polymer that holds everything together and grips the substrate. It sets most of the film’s properties — adhesion, flexibility, chemical resistance, weathering — and usually gives the coating its generic name: epoxy, polyurethane, acrylic, alkyd, polysiloxane, silicate. In reactive coatings the binder is formed on the surface from two or more components.
Pigments and extenders
- Prime pigments provide color and hiding: titanium dioxide for whiteness and opacity, iron oxides and organic pigments for color.
- Anticorrosive pigments work chemically or electrochemically: zinc dust protects sacrificially, while zinc phosphate and similar inhibitive pigments help passivate steel.
- Barrier pigments are platelet-shaped — micaceous iron oxide (MIO), glass flake, aluminum flake — and overlap like roof shingles, lengthening the path water and oxygen must travel.
- Extenders (fillers) such as barium sulfate, calcium carbonate, talc and silica adjust rheology, film build, sanding properties and cost.
Carrier
Solvent or water dissolves or disperses the binder so the coating can be applied at a workable viscosity. It leaves the film during drying, and organic solvent is the main source of VOC emissions. So-called 100%-solids products contain little or no carrier.
Additives
Additives are used in small amounts — often a few percent or less — but have outsized effects: wetting and dispersing agents, defoamers, anti-sag rheology modifiers, flow and leveling agents, UV absorbers and hindered-amine light stabilizers (HALS), catalysts, driers, flash-rust inhibitors in waterborne primers, and in-can preservatives.
Pigment volume concentration
The balance between pigment and binder is expressed as pigment volume concentration (PVC): the volume of pigment and extender divided by the total volume of non-volatile material in the dry film. As PVC rises, there is eventually just enough binder to fill the gaps between particles — the critical pigment volume concentration (CPVC). Above CPVC the film becomes porous, permeability rises sharply and gloss and strength fall.
High-gloss topcoats sit well below CPVC; flat architectural paints may sit near or above it. Barrier and inhibitive primers for steel are normally formulated below CPVC. Zinc-rich primers are a deliberate exception: they need a very high zinc loading so that particles touch each other and the steel, creating the electrical contact that sacrificial protection depends on.
The layers of a protective system
The prepared surface
Surface preparation is not a coat, but it is the foundation of every system. Cleanliness (removal of rust, mill scale, oil and soluble salts) and an appropriate surface profile determine whether the primer can adhere at all.
Primer
The primer’s jobs are adhesion to the substrate and, on steel, corrosion control. Options include zinc-rich primers (organic or inorganic) for sacrificial protection, inhibitive primers, and surface-tolerant epoxy mastics for less-than-ideal preparation. On concrete, a low-viscosity penetrating primer seals porosity and reduces outgassing.
Stripe coat
Edges, corners, welds, bolts and crevices are where film thins as wet coating pulls away. A brush-applied stripe coat on these areas, usually before or between full coats, is a cheap and effective way to extend system life.
Intermediate (build) coat
Usually a high-build epoxy, sometimes pigmented with MIO or glass flake, the intermediate coat adds thickness and barrier resistance and provides a compatible surface for the finish.
Topcoat
The finish faces the environment: sunlight, weather, chemicals, abrasion, cleaning. Aromatic epoxies chalk in UV light, so exterior steel typically receives an aliphatic polyurethane, polysiloxane or fluoropolymer topcoat for color and gloss retention.
Typical coating systems
The table shows common generic systems with indicative film thickness ranges. Actual values come from the specification and the product data sheets.
| System | Layers (typical DFT per coat) | Typical use |
|---|---|---|
| Zinc / epoxy / polyurethane | Zinc-rich primer 2–4 mils (50–100 µm) + epoxy 4–8 mils (100–200 µm) + aliphatic polyurethane 2–4 mils (50–100 µm) | Bridges, structural steel, coastal and industrial exposure |
| Epoxy / polyurethane | Epoxy primer or mastic 4–8 mils (100–200 µm) + aliphatic polyurethane 2–4 mils (50–100 µm) | Moderate industrial atmospheres, equipment |
| Multi-coat epoxy lining | Two or three epoxy coats, total commonly 12–20 mils (300–500 µm) | Tanks, immersion service, containment |
| Alkyd shop system | Alkyd primer + alkyd enamel, total roughly 3–6 mils (75–150 µm) | Mild interior or rural exposure |
| Concrete floor | Penetrating epoxy primer + epoxy body coat (often with broadcast aggregate) + polyurethane or polyaspartic topcoat | Industrial and commercial floors |
| Polyurea membrane | Primer + plural-component polyurea, often 40–125 mils (1–3 mm), with optional aliphatic topcoat | Liners, waterproofing, containment |
Compatibility rules
Layers must be chemically and mechanically compatible. A few generic rules recur across manufacturers:
- Strong solvents over weak films. Two-component epoxies or urethanes applied over oxidatively cured alkyds can lift and wrinkle them.
- Alkyds over zinc. The alkaline zinc surface saponifies the oil-based binder, producing soaps at the interface and eventual peeling.
- Porous inorganic zinc. Topcoats applied at full thickness over inorganic zinc can bubble and pinhole as air escapes from the pores; a thinned mist coat or tie coat is used first.
- Recoat windows. Exceeding a product’s maximum recoat time can leave the next coat with poor adhesion unless the surface is abraded or a tie coat is used.
Mixing products from different manufacturers within one system shifts compatibility responsibility to whoever made that choice. Unless a specification says otherwise, use a complete system from one manufacturer and follow its recommended coat sequence.
Film thickness: getting each layer right
Specifications state a minimum and maximum dry film thickness (DFT) for each coat and for the total system. Too thin, and the barrier is weak and holidays are likely. Too thick, and solvent becomes trapped, internal stress rises, and some products crack — over-thick zinc-rich primers are notorious for mud cracking. DFT is verified with calibrated gauges, with acceptance usually judged against SSPC-PA 2 or an equivalent sampling standard; see dry film thickness measurement.
When comparing bids or proposals, line up the generic type, number of coats and DFT range for every layer — not just the brand names or the total thickness. Two “10-mil systems” can perform very differently depending on how that thickness is built.
Designing systems with ISO 12944
For steel structures, ISO 12944 is the most widely used framework. Part 2 classifies atmospheric environments from C1 (very low) to C5 (very high) and CX (extreme), plus immersion categories Im1–Im4 (see ISO 12944 corrosivity categories). Part 5 lists example systems by environment and durability range: low (up to 7 years), medium (7–15 years), high (15–25 years) and very high (more than 25 years). Durability here means the expected time to first major maintenance — a planning figure, not a guarantee. Part 6 covers laboratory performance testing, and Part 9 addresses offshore and similar CX and Im4 structures.
Frequently asked questions
Do I always need a primer?
Not always. Some direct-to-metal and self-priming products are designed as single-coat or same-product multi-coat systems. For demanding environments, however, a dedicated primer chosen for adhesion and corrosion control is standard practice.
Why use an intermediate coat instead of a thicker topcoat?
Intermediate epoxies build thickness and barrier resistance economically, while UV-stable topcoats are optimized for appearance and weathering and are usually applied relatively thin. Splitting the jobs gives better performance per unit cost.
What is a tie coat?
A tie coat is a thin intermediate layer that bridges two otherwise incompatible materials — for example, sealing a porous inorganic zinc primer or linking an old coating to a new one in maintenance work.
Is a thicker system always more durable?
Only within the limits the products are designed for. Exceeding maximum DFT can cause cracking, solvent entrapment and poor cure, so a correctly built system at specified thickness outperforms an over-applied one.
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.