When Should Specialist Coatings Be Used? Intumescent, Anti-Carbonation, and Bituminous Products

Summary

Specialist coatings occupy the intersection between painting and structural protection. They are specified by engineers, fire officers, and asset managers — not chosen at the merchant counter — and their correct application has direct consequences for fire safety, structural integrity, and asset life. Applying a specialist coating incorrectly (wrong DFT, wrong primer, incorrect substrate preparation, application outside temperature or humidity limits) can render a certified system non-compliant and potentially dangerous.

The three most common specialist coating categories encountered by UK tradespeople are: intumescent coatings for structural steelwork (required under Part B of the Building Regulations for fire resistance); anti-carbonation coatings for exposed concrete structures; and bituminous coatings for buried or submerged metalwork and below-DPC masonry. Beyond these, zinc-rich primers and chlorinated rubber coatings are encountered in heavy industrial, marine, and offshore contexts.

COSHH is a significant consideration for all specialist coatings — solvent levels are high, isocyanates are present in many two-component systems, and heavy metal-containing products (zinc, chromate) require careful handling.

Key Facts

• Intumescent paint purpose — Expands when exposed to heat (above approximately 120°C) to form a carbonaceous char (known as intumesc) that insulates the steel from the fire for a defined period (30, 60, 90, or 120 minutes)
• BS 476-21 — British Standard for fire resistance of load-bearing elements (columns, beams); specifies the test method for measuring fire resistance periods; intumescent systems are tested and certified to this standard
• DFT (dry film thickness) — The critical parameter for intumescent coatings; a 60-minute fire rating on a particular beam section (defined by Hp/A — the section factor) requires a specific DFT, typically in the range of 0.25–3mm depending on the product and section factor; insufficient DFT means the system is non-compliant
• Hp/A (section factor) — A measure of the steel section's exposed perimeter (Hp) divided by its cross-sectional area (A); higher section factors mean thinner sections that heat up faster; thin-section steel requires a thicker intumescent coating
• Thin-film vs thick-film intumescent — Thin-film products (water or solvent-borne) are the most common for internal steelwork; thick-film epoxy-based products are used for external or high-humidity environments
• Anti-carbonation coating standard — BS EN 1504-2: products and systems for the protection and repair of concrete structures; defines performance requirements for anti-carbonation coatings (CO2 diffusion resistance, equivalent air layer thickness)
• Carbonation depth — CO2 from the atmosphere reacts with calcium hydroxide in concrete, lowering the pH and destroying the passivating layer protecting reinforcement from corrosion; carbonation progresses inward at approximately 1–2mm per year in average UK conditions; reinforcement corrosion begins when the carbonation front reaches the steel
• Bituminous paint standard — BS 3416: specification for bitumen-based coatings for cold application; used for buried metalwork, underground pipework, galvanised tanks, and below-DPC masonry
• Zinc-rich primer — Contains a high proportion of zinc dust (typically >80% by weight in dry film); the zinc acts as a sacrificial anode, corroding preferentially to protect the steel; used as the primary corrosion protection primer in aggressive environments
• Chlorinated rubber — Solvent-based single-component coating; excellent chemical resistance; used for offshore structures, chemical plant, and swimming pools; being phased out in some applications due to chlorinated solvent concerns
• DFT measurement — Wet film thickness gauge used during application; dry film thickness measured with an electromagnetic or eddy-current DFT gauge after curing; accurate DFT records are required for certification
• Isocyanates — Many two-component specialist coatings (polyurethane, certain epoxies) contain isocyanate hardeners; isocyanate sensitivity causes occupational asthma — RPE rated for isocyanates (half-mask with OV/P filter) is mandatory; health surveillance required under COSHH

Quick Reference Table

Detailed Guidance

Intumescent Coatings for Structural Steelwork

Intumescent coatings are the primary method of achieving fire resistance for exposed structural steelwork in modern UK construction. The alternative approach — boxing the steel in plasterboard or sprayed mineral fibre (vermiculite) — is less aesthetically acceptable for many contemporary projects.

How they work: Intumescent coatings contain three key chemical components: a carbon-forming ingredient (a carbonific), an acid catalyst, and a blowing agent. When the coating reaches approximately 120°C in a fire, the acid catalyst decomposes and attacks the carbonific, forming a carbonaceous foam. The blowing agent simultaneously releases a gas that expands this foam to many times its original volume. The resulting char is a poor thermal conductor and insulates the steel beneath, slowing the temperature rise of the steel and maintaining its structural integrity for the rated period.

Section factor (Hp/A): Steel sections heat up at a rate proportional to their surface area to mass ratio. A thin flat plate heats up much faster than a heavy universal column. The section factor (Hp/A, measured in m⁻¹) quantifies this — typically ranging from 50 m⁻¹ (heavy sections) to 300+ m⁻¹ (thin sections). A given product will require a greater DFT for high section factors to achieve the same fire period rating. The intumescent manufacturer provides a DFT schedule correlating section factor, fire period, and required DFT for their product.

Application: Thin-film intumescent can be applied by brush, roller, or airless spray. Airless spray is standard for commercial work — it produces a more consistent film build and is significantly faster on large projects. The product must be applied at the correct WFT (wet film thickness) to achieve the specified DFT — wet film gauge checks at frequent intervals are essential.

Quality assurance: For fire-rated structural steel work, DFT records are required throughout the project. The number of measurement readings per surface area is specified in BS EN ISO 19840 [verify]. Non-compliant areas (under DFT) must be identified, documented, and overcoated. The completed system must be certified by the applicator and witnessed by the structural engineer or fire engineer where required.

Internal vs external: Standard thin-film intumescent coatings are formulated for internal use in dry, conditioned environments. For exposed steel in car parks, external structures, or high-humidity environments, epoxy-based intumescent systems are required — these are significantly more expensive and harder to apply, but provide the necessary resistance to moisture and weathering.

Anti-Carbonation Coatings for Concrete

Carbonation is a progressive chemical process in which atmospheric CO2 reacts with calcium hydroxide in the concrete's cement paste, converting it to calcium carbonate and reducing the pH from approximately 12 to below 9. When the carbonation front reaches the steel reinforcement, the protective oxide layer on the steel breaks down and corrosion begins. In many UK concrete structures built in the 1960s–1980s with inadequate cover depths, carbonation has already reached the reinforcement.

Anti-carbonation coatings work by creating a barrier that slows the diffusion of CO2 into the concrete. They are characterised by their equivalent air layer thickness (sd value) — a coating with sd of 50m (for example) retards CO2 diffusion as effectively as 50 metres of open air. For significant carbonation protection, an sd value of at least 50–100m is typically required.

Application context: Anti-carbonation coatings are typically specified as part of a concrete repair scheme designed by a structural engineer, following survey and carbonation depth testing (phenolphthalein indicator test). They are applied to sound, clean concrete — often after patch repair of spalled or cracked areas using a compatible repair mortar.

Surface preparation: The concrete surface must be clean, sound, and free from contamination. Laitance, carbonated surface concrete, and loose material are removed by high-pressure water jetting or grit-blasting. Oil and chemical contamination must be removed chemically. The surface should be dry (surface moisture content below the threshold specified for the product — typically 5–6% by weight).

Application: Typically applied in two or three coats by brush, roller, or airless spray to achieve the specified DFT. The first coat is often a sealer to penetrate and consolidate the concrete surface. Check for pinholes and holidays (uncoated areas) — these are entry points for CO2 and significantly reduce the effectiveness of the system.

Bituminous Coatings

Bitumen-based coatings (bituminous paints) are thermoplastic, black, solvent-borne products used primarily to protect metal and masonry in below-grade or permanently wet applications — buried pipework, underground tanks, below-DPC masonry, and water storage tanks. They are inexpensive, easy to apply, and effective against water, biological attack, and mild chemical exposure.

BS 3416 specifies two types: Type 1 (applied cold, for general use) and Type 2 (applied heated, for more demanding applications). Cold-applied products are the norm for maintenance and construction site applications.

Application: Apply to clean, dry, blast-cleaned or mechanically prepared metal, or to clean dry masonry. Two coats minimum, each allowed to dry before the next. Total DFT typically 100–200 microns. For buried applications, a holiday detector (spark tester) should be used to identify pinholes before backfilling.

Limitations: Bituminous coatings are not suitable for surfaces exposed to strong solvents, fuels, or oils. They soften in direct sunlight (they should be covered in buried applications, not left exposed). They are not suitable for potable water applications unless specifically certified for water contact.

DFT Measurement and Inspection

For all specialist coatings, dry film thickness measurement is the primary quality control activity. The tools are:

• Wet film gauge: A simple comb gauge placed into the wet film during application; indicates whether the applied WFT is correct for the target DFT (DFT ≈ WFT × solids-by-volume percentage)
• Electromagnetic DFT gauge (for ferrous substrates): Uses the magnetic attraction between a probe and the underlying steel, reduced by the presence of a non-magnetic coating; accurate on blast-cleaned steel
• Eddy-current DFT gauge (for non-ferrous substrates): Uses eddy current induction; used for coatings on aluminium, zinc (galvanised), and other non-ferrous metals

Take a statistically representative number of readings — at minimum, one reading per m² for intumescent coatings, with additional readings at plate edges, around welds, and at connections. Low spots below the minimum DFT must be overcoated. Records of all DFT readings should be retained as part of the project quality documentation.

Frequently Asked Questions

Who can apply intumescent coatings for fire resistance?

Intumescent coatings for structural steel fire protection should be applied by a contractor trained and certified in the application of the specific product system. Many product manufacturers require licensed applicators. On fire-rated steel, the certifying body (structural engineer, fire engineer, or building control) will want evidence that the application was carried out by a competent person with documented DFT records.

Can I apply anti-carbonation coating as a preventive measure on new concrete?

Yes, as a maintenance measure, anti-carbonation coatings are sometimes applied to new exposed concrete structures in aggressive environments (coastal, urban with high CO2 pollution, industrial) before carbonation has progressed. This extends the maintenance-free life significantly. However, the concrete must still be properly cured, finished, and allowed to gain strength before coating.

Is bituminous paint safe for drinking water tanks?

Standard BS 3416 bituminous paint is not approved for potable water contact. For water tanks, cisterns, or pipes containing drinking water, use a product specifically approved to BS 6920 (suitability of non-metallic products for use in contact with water intended for human consumption) or WRAS-approved coatings.

What PPE is required for applying two-component polyurethane coatings?

Two-component polyurethane coatings contain isocyanate hardeners. Isocyanates are one of the leading causes of occupational asthma in the UK. Mandatory PPE: half-face respirator or full-face mask with organic vapour and P3 combination filters; protective gloves (nitrile, not latex); chemical-resistant overalls; eye protection. Health surveillance (baseline lung function test) should be established before first exposure and repeated annually. Spray application should only be carried out in a suitable enclosure with adequate ventilation.

Regulations & Standards

• BS 476-21 — Methods for determination of the fire resistance of loadbearing elements of construction; fire testing standard for structural elements including intumescent-coated steel

• BS EN 1504-2 — Products and systems for protection and repair of concrete structures; anti-carbonation and surface protection coatings

• BS 3416 — Specification for bitumen-based coatings for cold application; type definitions and performance criteria

• BS EN ISO 12944-5 — Paints and varnishes: corrosion protection of steel structures by protective paint systems; coating system selection for exposure categories

• COSHH Regulations 2002 — Risk assessment, exposure monitoring, and health surveillance for isocyanate-containing products

• HSE EH40 — Occupational exposure limits, including for isocyanates and solvents [verify current edition]

• Association for Specialist Fire Protection (ASFP) — Technical guidance on intumescent systems and fire protection specification

• Sherwin-Williams / International Paints — Intumescent product data — DFT schedules and application guidance

• Concrete Society — Concrete repair and protection — Anti-carbonation coating specification

• HSE — Isocyanates health guidance — COSHH, health surveillance, and RPE for isocyanate exposure

• Jotun UK — Specialist coating technical literature — Zinc-rich and heavy-duty coating systems

• exterior paint preparation — Surface preparation fundamentals applicable to specialist coatings

• paint types guide — Standard decorative paint types and when specialist products are needed