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Steel Beam Painting, Priming and Intumescent Coating

Primer, decorative topcoats and intumescent fire protection, applied before the beam is delivered rather than around it once it is installed.

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Primer is protection in transit, not for life

Structural steel is commonly supplied with a coat of primer, and it is worth being clear about what that does. Primer protects the steel while it is being handled, stored and transported, and gives a topcoat something to key to. It is not a long-term corrosion system, and a primed beam left exposed to the weather will still rust.

For a beam that is going to be boxed in and live in a dry, heated interior, primer is usually all it needs. For anything that will be seen, or anything that will be damp, the specification needs to go further.

Intumescent coating and fire protection

Steel loses strength as it heats, so where a beam is left exposed rather than encased, the Building Regulations may require it to be protected so that it holds its load for a specified period in a fire. On domestic work this comes up most often in loft conversions and open-plan conversions where the steel is a visible part of the finished room.

Intumescent coating is the usual answer where the steel is meant to be seen. It looks like paint, and in a fire it swells into an insulating char that slows the rate at which the section heats. The film thickness required is not a matter of taste — it depends on the fire resistance period required and on the section’s own shape, so it has to come from the specification rather than from a tin.

The alternative, where appearance does not matter, is encasement in plasterboard. If your beam is being boxed in anyway, ask whether that already satisfies the requirement before paying for a coating.

Decorative finishes

Exposed steel in a finished room is now a deliberate design choice as often as a structural necessity, and a beam can be delivered in a topcoat rather than primer. Getting the finish applied before delivery generally gives a better result than painting a beam in situ over people’s heads, and it avoids masking a finished room.

Tell us the colour reference and whether the finish needs to be compatible with an intumescent system underneath, because not every topcoat is.

What we need from you

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 any accompanying details.

Tell us as well if any part of the beam is to be welded on site, so those areas can be 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 any site welding is planned, tell us 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.

Ready for a price?

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.