Paints for iron and metal: rust-inhibiting primer, zinc-rich coating or enamel?

For iron that rusts outdoors you need a two-stage system: a rust-inhibiting primer (or a zinc-rich coating) to stop corrosion and a finish that withstands sun and rain. For a quick touch-up or a small part a 400 ml spray is enough; for railings, structures and roofing, a tin of 0.5, 2.5 or 16 litres applied by brush, roller or spray is the better choice. If the surface is exposed to chemicals or impacts, the right finish is a polyurethane or acrylic-polyurethane enamel.

Types

• Rust-inhibiting primers and primers: these are the undercoats. In the collection you will find synthetic, oleo-phenolic and undercoat rust-inhibiting primers. They are applied on clean iron before the finish, such as the synthetic rust-inhibiting primer Verfer Nitro or the Verfer NS grey base primer.
• Zinc-rich coatings: light zinc, pure zinc and zinc spray, in 400 ml cans. They give protection similar to cold galvanising and are good for welds, cuts and touch-ups on galvanised iron. An example is the Light zinc Fervi.
• Micaceous iron oxide finishes: they have a matt appearance similar to wrought iron and are used on gates, railings and structures. An example is the Zincover micaceous finish.
• Polyurethane and acrylic-polyurethane enamels: hard finishes, including water-based or two-component ones, for surfaces exposed to weather and chemicals. An example is the two-component enamel Marco Polo 2K.
• Aluminium paints: for metal roofing, such as Allumisol 12/14.
• Blackening agents and protective coatings: cold blackening agents are applied on bare metal (steel, iron, copper, brass) and rinsed as indicated on the label. Among the surface protectives is Bronzilux.

In practice the system has three layers: preparation, primer, finish. Preparation removes rust and grease, the primer (rust-inhibiting or zinc-rich) protects the iron, the finish protects the primer and gives colour. Some products act as primer and finish at the same time, but they are less suitable for aggressive environments: always read the data sheet of the product you choose.

How to choose for the job

Features to check

• Application: in the collection you will find products for brush, roller, spray and airless. If you have a lot of surface to cover, choose one that can also be applied by spray or airless.
• Type: single-component (used straight from the tin) or two-component (to be mixed with the hardener and used within a limited time). Two-component is more resistant, but requires more care.
• Resistance: the data sheets refer to resistance to weathering, to corrosion and oxidation, to chemicals and to impacts. For outdoor use weather resistance is enough; for workshops and plants resistance to chemicals and impacts is needed.
• Material: polyurethane, water-based acrylic-polyurethane, synthetic. Water-based enamels have less odour, while solvent-based ones remain widely used on site.
• Pack: 0.5, 0.75, 2.5 and 16 litres, or 400 ml spray. Work out the surface area and the coverage stated on the data sheet before choosing the size.
• Colour and appearance: grey, black, silver, clear; light zinc or yellow/gold; matt wrought-iron look.

Common mistakes

• Painting over rust without preparing it. Wire brushing or abrasive paper and degreasing are not optional: rust under the finish keeps on corroding.
• Skipping the primer. Many finishes are not rust-inhibiting: without the primer, the protection does not last long.
• Mixing incompatible products. A synthetic primer and a water-based enamel (or vice versa) may not adhere: check the permitted systems on the data sheet.
• Applying one thick coat only. Thin, repeated coats are better, observing the stated drying times.
• Painting in rain or damp conditions. Wait until the product has dried properly, otherwise the film is ruined.

Frequently asked questions

Can I paint rusty iron directly?

Only after removing the rust that comes off with a wire brush or abrasive paper and degreasing. Then apply the rust-inhibiting primer or zinc-rich coating, never the finish alone.

What is the difference between a zinc-rich coating and a rust-inhibiting primer?

A rust-inhibiting primer is an undercoat that hinders corrosion. A zinc-rich coating contains zinc and protects like cold galvanising: it is used where the metal is highly exposed or on welds and cuts.

Which enamel for a structure that stays outdoors?

A polyurethane or acrylic-polyurethane enamel, chosen for its resistance to weathering. If the structure also comes into contact with chemicals, check that the data sheet covers this.

Is spray suitable for large surfaces?

No, the 400 ml can is designed for touch-ups and small parts. For large surfaces, a tin applied by roller, brush or spray is the better choice.

How many coats are needed?

Usually one or two of primer and one or two of finish, always thin. The exact number and the recoating times are on the data sheet.

You can find the whole range in the paints for iron and metal collection. If you are unsure about the system to use, ask the AMES technicians for a quote or advice.