Alkyd paint systems

What an alkyd system is

Alkyd paints are single-pack coatings based on oil-modified polyester resins. They dry first by solvent evaporation and then cure by reaction with oxygen in the air (oxidation). No mixing is needed, pot life is not an issue and the paint is tolerant of application conditions, so alkyds are quick to apply in the factory, easy to touch up on site and simple to overcoat during maintenance. Their low cost makes them the standard shop primer for many pre-engineered buildings where the frames sit inside dry, enclosed buildings.

The trade-off is durability. Alkyd films are thinner and less dense than two-component epoxies and are softened by alkalis, solvents and constant moisture. They belong in the lower corrosivity categories; for harsher environments see epoxy systems, hot-dip galvanizing or the overall corrosion protection guide.

Where alkyd is appropriate

Typical uses are warehouses, factories and interior mezzanine steel, as well as misc. metals hidden behind finishes. Choosing the corrosivity category correctly matters most: an interior that is unheated, poorly ventilated or near the coast may be C3 or higher, and then alkyd is no longer the economical choice over the life of the building.

Typical build-ups and application

• Single alkyd primer – for steel that will be hidden in a dry, heated building, or that will be finish-painted on site by others.
• Alkyd primer + alkyd finish – a two-coat system where the frames remain visible inside the building and a colour finish is required.
• Zinc phosphate alkyd primer – an anti-corrosive pigment variant used as a primer in many specifications.

World Steel abrasive blast cleans the steel before priming – typically to Sa 2 or Sa 2½ (ISO 8501-1), or SSPC-SP 6 / SP 10, as the specification and data sheet require – and applies the paint by airless spray, with brushed stripe coats on edges and welds. Film thickness per coat and drying times follow the manufacturer's data sheet; thick alkyd films dry slowly and can wrinkle, so build is achieved in coats rather than one heavy application. Total system thickness and the number of coats for each category and durability are taken from EN ISO 12944-5 or AS/NZS 2312.1 and confirmed with the manufacturer. Background on shop painting is in the PEB articles on paint and coating systems and surface preparation.

Limitations and compatibility

• Zinc surfaces: alkyd must never be applied directly to galvanized or zinc-rich surfaces – the alkaline zinc products saponify the binder and the paint peels. Use a duplex system with a compatible primer instead.
• Overcoating with stronger coatings: two-component epoxies and polyurethanes contain strong solvents that can lift an alkyd film; overcoating an alkyd with them requires the manufacturer's confirmation.
• Concrete contact and immersion: alkyd is attacked by alkali from wet concrete and is not suitable for immersion or buried steel.
• Fire protection: an alkyd primer may sit under an intumescent coating only when the fire-protection manufacturer has tested and approved that primer for the product.
• Handling: alkyd cures slowly in thick films and in cool weather, so packing is scheduled after adequate drying to avoid sticking and damage in transit.

Selecting and specifying

Specify the corrosivity category, durability and whether the steel is visible. State the colour (RAL, AMS-STD-595 or AS 2700) where a finish coat is needed, identify any members to be galvanized or fire-protected, and confirm whether site touch-up and finish painting are in or out of World Steel's scope. Where the client has a preferred manufacturer, World Steel applies that manufacturer's alkyd system according to its data sheet and the project specification.