Wet Film Thickness (WFT) Explained

The Required DFT Is Member-SpecificFire Resistance Is Thickness-Dependent

Intumescent products are tested and assessed across defined steel sections, exposure conditions and fire-resistance periods. The required DFT for an individual member is taken from the accepted product or system data using the relevant project inputs.Fire resistance ratings — such as 30, 60 or 90 minutes — are achieved through independent laboratory testing of intumescent coating systems applied at specific thicknesses. These tests establish how long coated steel remains below its critical failure temperature during a fire.

WFT is an application target used to work towards that required DFT. The relationship depends on the particular product and its volume-solids or manufacturer conversion information. Under-application can leave insufficient DFT, while excessive build or unsuitable recoating can introduce defects.

A Reading Taken While the Coating Is WetImmediate, In-Process Measurement

WFT is checked shortly after application using a suitable wet-film gauge and the method defined for the product and project. Readings provide immediate feedback but must be interpreted alongside coverage, coat limits and drying requirements.

This helps the applicator manage coating build before the material dries. It does not replace final DFT assessment.

What WFT Records ShowWhy WFT Logging Matters

WFT records show the wet coating build observed during application. They form part of the QA record and help explain how application was controlled.

WFT records alone do not establish the achieved fire-resistance period; that conclusion depends on the accepted design, system data, member schedule and final inspection information.

WFT control requires suitable equipment, product knowledge and an understanding of the accepted application target. It should sit within a wider process covering preparation, site conditions, coat limits, drying, DFT inspection and records.

WFT, relevant site conditions, products and work areas can be recorded during application. Final DFT and handover records then document the completed FireCoatings scope for project-team review.