Epoxy and polyurethane paint systems

What an epoxy system is

Epoxy paints are two-component coatings: a resin base and a curing agent are mixed before application and cure chemically into a hard, dense film with strong adhesion and good resistance to moisture, chemicals and abrasion. They are the backbone of most industrial and marine paint specifications for structural steel in C3, C4 and C5 environments. Because epoxies chalk and fade in sunlight, an exposed system is finished with a polyurethane (or polysiloxane) topcoat that holds colour and gloss.

A typical three-coat system has a distinct job for each layer, as shown below. For the choice between epoxy, galvanizing, alkyd and duplex, start with corrosion protection systems; for background on generic paint types see the PEB article on paint and coating systems.

Typical build-ups

The table is qualitative and the thickness ranges are approximate. Three-coat zinc-rich epoxy / epoxy / polyurethane systems with roughly 200–320 µm total nominal DFT are common for C4–C5 with high durability; every project system is confirmed against EN ISO 12944-5 (or AS/NZS 2312.1) and the manufacturer's data sheets.

• Zinc-rich epoxy primer – a high loading of zinc dust gives cathodic protection at scratches and edges; it needs a clean, near-white blasted surface.
• Epoxy or epoxy MIO intermediate – builds film thickness as a barrier; micaceous iron oxide (MIO) flakes lengthen the path for moisture and give a good key for the topcoat.
• Polyurethane topcoat – aliphatic polyurethane gives UV resistance, colour and gloss retention; polysiloxane is an alternative where specified.

Surface preparation and application conditions

Steel is abrasive blast cleaned in the factory to Sa 2½ (ISO 8501-1) / SSPC-SP 10 / NACE No. 2 for zinc-rich and most epoxy primers, with the surface profile set by the data sheet and checked with an ISO 8503 comparator. Welds and edges are prepared to the ISO 8501-3 grade specified (P1–P3), and dust and soluble salts are controlled where the specification requires. Australian work follows AS 1627.4.

Epoxies are sensitive to application conditions. World Steel follows the manufacturer's data sheet for mixing ratio, induction time, pot life, film thickness per coat and minimum and maximum overcoating intervals. Painting proceeds only when the steel temperature is at least 3 °C above the dew point and air temperature and relative humidity are within the data sheet limits; readings are logged. Stripe coats are brushed onto edges, welds, bolt holes and corners before each full coat, because spray films are thinnest there. More detail is in the PEB article on surface preparation and painting.

Inspection and handling

Dry film thickness is measured per coat and in total to ISO 19840 or SSPC-PA 2, depending on the market. Adhesion is checked where specified by pull-off testing (ISO 4624 / ASTM D4541) or cross-cut / tape tests (ISO 2409 / ASTM D3359), and the film is inspected visually for runs, sags, pinholes, dry spray and contamination. Results go into the coating inspection report. Fully cured members are packed with soft slings, timber dunnage and edge protection; contact surfaces of slip-critical bolted joints are masked or coated only with a qualified slip coating as the specification demands.

Epoxy systems are commonly specified on pipe racks, equipment support structures and platforms where the steel must match the plant colour scheme. Systems from manufacturers such as Jotun, Hempel and International Paint are applied at World Steel according to the project specification.

Selecting and specifying

• State the corrosivity category and durability, or the full system with generic types, number of coats and nominal DFT.
• Name the approved manufacturer(s) or ask World Steel to propose an equivalent system with data sheets.
• Give the topcoat colour standard (RAL, AMS-STD-595 or AS 2700) and gloss level.
• Identify faying surfaces, members to receive fire protection and areas to be left uncoated for site welding.
• Agree the touch-up procedure and who supplies touch-up paint for erection damage.