Steel Beam Painting, Priming and Intumescent Coating

Primer, topcoats and intumescent fire protection, applied before the beam is delivered instead of around it once it is installed.

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Primer is protection in transit, not for life

Structural steel usually arrives with a coat of primer on it, and it is worth being clear what that coat is actually for, because primer protects the steel while it is being handled, stored and transported, and it gives a topcoat something to key to, and that is the whole of its job. It is not a corrosion system. Leave a primed beam out in the weather and it will rust.

For steel that will be boxed in and spend its life in a dry, heated interior, primer is usually all it needs. Anything visible needs more. So does anything damp.

Intumescent coating and fire protection

Steel loses strength as it gets hot, so where a beam is left exposed instead of encased, the Building Regulations may require it to hold its load for a set period in a fire. On domestic work this comes up most often in loft conversions and open-plan conversions, where the steel has ended up as a visible part of the finished room.

Intumescent coating is the usual answer when the steel is meant to be seen. It goes on looking like paint, and in a fire it swells into an insulating char that slows the rate at which the section heats. Thickness is not a matter of taste. It depends on the fire resistance period required and on the shape of the section itself, both of which come off the specification.

Where appearance does not matter, encasing the steel in plasterboard does the same job. If your beam is being boxed in anyway, it is worth asking whether that already satisfies the requirement before anybody pays for a coating.

Decorative finishes

Exposed steel in a finished room is a deliberate design choice as often as a structural necessity these days, and a beam can be delivered in a topcoat instead of primer. Applying the finish beforehand generally gives a better result than painting a beam in situ over people’s heads, and it saves masking out a room that is already finished.

Give us the colour reference. Tell us as well if the topcoat has to sit over an intumescent system, because not every finish is compatible with one.

What we need from you

We need the specification. Primer only, primer plus topcoat, or an intumescent system with a required fire resistance period. If your engineer or building control officer has specified a period, quote it to us and send whatever details came with it.

Tell us if any part of the beam is to be welded on site, so those areas are left uncoated.

Do not coat areas that still have to be welded. Paint, primer and intumescent all have to be kept off surfaces that will be welded later, and coatings near a weld zone can be damaged by the heat. If site welding is planned, say so at quote stage so those areas are masked.

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Common questions

Does a steel beam have to be painted?

Not always. A beam boxed into a dry interior is often fine in primer alone. Beams that are exposed, external or in damp conditions need a proper coating system or galvanising.

When do I need intumescent coating?

Where a beam is left exposed and the Building Regulations require it to maintain its load-bearing capacity for a set period in a fire. Your building control officer or engineer will specify the period required.

Is intumescent coating an alternative to boxing in?

It is the usual option where the steel is meant to be visible. If the beam is being encased in plasterboard anyway, check whether that already meets the requirement before paying for a coating.

Can I paint the beam myself after installation?

You can, but the finish is generally poorer and the work far more awkward once the beam is overhead in a finished room. Anything specified for fire protection should be applied to the specification rather than by eye.

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Send your drawings or a cutting list and we’ll come back with quantities and a price, steel and processing together. Large and small orders both welcome.

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We supply structural steel across London, the South and the Midlands, delivered direct to site. Information on this page is general guidance; specifications for your project must come from your structural engineer.