Wet film thickness (WFT) is the thickness of freshly applied coating before water or solvent has evaporated. Taking WFT readings gives the applicator immediate information about the amount placed in a pass. It is an application-control tool, not the final acceptance measurement and not proof of the structural fire performance of the completed system.
Key points
- Calculate the target WFT using the selected product’s solids information and specified dry-film build.
- Take readings promptly and at representative locations without damaging important finished areas unnecessarily.
- Use results to adjust application while the coat is wet.
- Assess final dry-film thickness only after the coating has dried sufficiently and in accordance with the agreed inspection method.
Why WFT is measured
Spray application can produce variations around flanges, webs, edges and connections. WFT readings help identify whether a pass is broadly within its intended range before it dries. This can reduce underbuild, excessive local thickness and avoidable additional coats.
The target per pass must remain within the product’s maximum and minimum application requirements. The final specified build may require several passes.
Converting dry-film requirement to wet-film target
A simplified calculation uses the required dry-film thickness divided by the coating’s volume-solids fraction. For example, a nominal 70% volume-solids product would require more wet material than the eventual dry build. The current product data must be used because published values, tolerances and calculation conventions can vary.
Thinning, if permitted at all, can change the relationship. It should never be introduced without following the manufacturer’s instructions and accounting for its effect on application control.
Using a wet-film comb
A clean gauge is pressed squarely into the wet coating and withdrawn. The reading lies between the highest tooth that is coated and the next tooth that remains uncoated. The gauge range and scale should suit the expected build.
Readings should be taken soon after application, before surface drying changes the result. The operator should avoid sliding the gauge or pressing it into edges where the reading is unrepresentative.
Choosing representative locations
The agreed frequency should reflect the member geometry, application method, coat and project inspection plan. Flanges, webs, columns, beams and complex connections may need separate consideration.
A few convenient readings cannot describe an entire area. Results should be linked to identifiable locations or work zones and reviewed for patterns, not treated only as isolated numbers.
Recording and responding to results
A useful record can include project area, member or grid reference, date, product, coat number, target range, readings and operative. Environmental records may be held alongside it where required.
If readings are consistently low or high, application should be adjusted. An excessive pass should be assessed against the maximum per-coat build and drying requirements rather than assumed acceptable because the final total needs more material.
WFT versus DFT
WFT predicts the likely dry build; dry-film thickness (DFT) is measured after appropriate drying. The relationship is affected by solids content, application variation, surface geometry and measurement method.
Final DFT inspection should use the specified procedure, calibrated equipment and applicable product or project acceptance criteria. Our WFT guide provides a concise site reference. The ASFP technical-document library also contains industry guidance on intumescent application and control.
Frequently asked questions
Does a correct WFT reading prove the final DFT is correct?
No. It predicts and controls application. Final DFT must be assessed after drying using the agreed method.
How many WFT readings are required?
There is no single number for every project. Frequency should reflect the product, member geometry, application area and agreed inspection plan.
Can an applicator average high and low readings?
Averages can be useful, but local limits and acceptance criteria also matter. Significant thin or excessive areas should not be hidden by an overall average.
Discuss your project
For on-site application with an agreed inspection scope, send Fire Coatings the coating schedule and project requirements.
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