Intumescent paint for structural steel

How intumescent coatings work

When heated, an intumescent coating expands many times its applied thickness into a foam-like char that insulates the steel. A system is usually a compatible primer, the intumescent basecoat and a sealer or decorative topcoat that protects it from moisture and damage. Each layer must be compatible with the others, so products are not mixed between systems.

Fire rating and thickness

The required intumescent paint rating, commonly a period such as 30 or 60 minutes, comes from the building's fire strategy. The dry film thickness needed to meet it depends on the steel section size and shape and on whether it is a beam or column, and the coating manufacturer calculates it. Thickness is then checked with a gauge during application. A coat that looks fine but is too thin will not perform.

• Specification from the fire strategy
• Compatible primer, basecoat and topcoat
• Thickness checked during application
• Records of products and readings

Internal and exterior intumescent paint

Most intumescent systems are designed for dry internal conditions. Exterior intumescent paint, or steel in damp or exposed areas, needs a system rated for that environment and a suitable sealer coat. Using an internal product outside can lead to breakdown long before anyone notices.

Preparation and application

Steel is cleaned and prepared to the manufacturer's requirements, existing coatings are checked for compatibility, and the basecoat is applied in passes by spray or brush to build the required thickness. On occupied or live sites the work is phased, with RAMS supplied and dust and overspray contained. We are fully insured and health and safety compliant.

Intumescent paint cost

Cost depends mainly on the total steel surface area, the section sizes, the fire rating and therefore thickness required, access height and the condition of any existing coating. A higher rating on slender sections needs considerably more material and time than a lower rating on heavy sections. Share the fire strategy or steel schedule and we will arrange the specification and a free quote.

Related reading

Our guide to intumescent paint explains the basics in plain terms. For railings and decorative steelwork that do not need a fire rating, see metalwork painting.

Inspection and maintenance

Once applied, an intumescent coating must stay intact to work. Mechanical damage, later services being fixed through it, or moisture getting in can all reduce performance. Building managers should keep the product and thickness records with the fire safety file, check exposed steel during routine inspections, and repair damage with the same system rather than a general paint. Where a building is being refurbished, existing coatings are identified first so any new work is compatible.

Working alongside other trades

Intumescent work is often programmed during fit-out, after steel is erected and before ceilings close. We coordinate with the main contractor so areas are released, coated and checked before they are covered up.