Firstly, let’s consider how intumescent paint works and what exactly lies beneath the surface of these seemingly ordinary coatings.
Unprotected structural steel begins to lose its structural capacity at around 520 degrees Celsius for columns or 620 degrees Celsius for beams in accordance with BS476, not taking steel loading into consideration (which could increase or decrease this limiting temperature). In a fire scenario, this temperature can be reached in about 10-15 minutes.
Intumescent paint coatings represent a form of passive fire protection. When subjected to high temperatures, these paints undergo a transformative process known as intumescence, causing them to expand. This expansion, often at a rate of 50:1 for thin-film coatings and 5:1 for thick-film coatings, results in the formation of a char layer. This char acts as a barrier, insulating the underlying substrate (usually steel) from the fire’s heat.
This process of absorbing thermal energy and forming a protective barrier buys invaluable time, delaying structural failure. It allows occupants more time to evacuate and firefighters more time to contain the fire, ultimately aiding in bringing the situation under control.
The application process of intumescent paints demands precision and adherence to specific guidelines. Proper surface preparation, mixing, and application techniques are essential to ensure optimal performance.
Considerations for application
Various factors come into play when applying intumescent paint, depending on the environment in which it will be used. Whether it’s an office building, a car park, or a swimming pool, each setting has distinct requirements for product selection including topcoats and application techniques, categorised into C1, C2, C3, C4, C5 and C5M environments.
Specifying the correct paint system is critical; intumescent coatings are not applied in isolation, but as part of a full paint system. A primer is required to provide the steel with corrosion protection, followed by the ‘base coat’ which is the intumescent coating and an optional top coat. The intumescent coating dry film thickness (DFT) must be pre-defined to ensure the correct level of protection is achieved. This is determined by several factors including: the fire resistance period, the testing standard, the steel section factor (A/V) and the number of sides of the steel section exposed.
Prior to applying the primer, the steel surface must be prepared to the correct standard specified by the paint manufacturer, typically SA 2.5. This ensures the primer adheres properly to the steel surface and provides the required corrosion protection for the lifetime of the paint system.
Before beginning the intumescent paint application procedure, it’s crucial to verify the compatibility of the primer with the manufacturer’s intumescent paint if the primer was shop applied. Additionally, you need to check the dry film thickness (DFT) of the primer. Obtaining the RAL number of the topcoat and determining the required fire rating are essential steps before proceeding.
Furthermore, it’s vital to determine if the building will be watertight before application, as this could impact the effectiveness of the intumescent paint product, particularly if you are using a solvent- or water-based product.
Application Procedure
Precision is key during the application of intumescent paint. Rushing is not an option. It’s essential to conduct a thorough site inspection beforehand, involving primer checks, performing dry film thickness (DFT) measurements, and rust spot inspections. Proper cleaning of steel beams are vital steps in the preparation process.
Furthermore, you’ll need to monitor the weather conditions, including ambient temperature, steel temperature, dew point and humidity, daily before and during application to ensure they remain within the manufacturer’s specified guidelines.
Calculating the required DFT for different steel section sizes is essential to achieve the desired fire rating. When determining the required DFT thickness, it’s essential to calculate how many applications may be needed to achieve the desired thickness, considering the maximum thickness allowed per coat as per the manufacturer’s instructions. For instance, if the required DFT is 4000 microns, it may necessitate four visits or more to spray the steel sections to achieve the required thickness, which must be accounted for in the quotation.
Identifying galvanised sections is important, as they require a different preparation process before intumescent application. Galvanised sections necessitate mordant wash and cleaning followed by the application of a two-part primer, which requires drying time specified by the manufacturer period before intumescent application.
Furthermore, it’s important to ascertain if the coated beams/columns require a high decorative finish, as this can impact costs and prolong the application process.
During application, a wet film thickness gauge (WFT) is used to monitor thickness. After achieving the desired thickness, a calibrated electronic thickness gauge verifies the DFT. Once confirmed, the topcoat is applied, and upon completion, masked areas are removed.
Finally, all work should be UKAS Third Party Certified and the applicator should provide a ‘golden thread of information’ on the completed work in compliance with Regulation 38 of the Building Regulations and the Fire Safety Act.
In summary, intumescent paint provides critical fire protection, but its successful application demands meticulous planning and execution to ensure effectiveness, competence, fire safety and durability.