Wet and dry film thickness: check each intumescent coat against its plan
A tower's coating crew is applying by feel while the first dry-film survey flags thin locations. The lead needs a documented coat plan and meaningful wet-film records.
Use the coating's actual volume-solids information and application instructions to establish the planned wet-film check for each coat. The basic relationship is dry film thickness equals wet film thickness multiplied by the volume-solids fraction. It is a planning calculation, not an acceptance test. Required total protection thickness, permitted coat build and final dry-film verification remain separate requirements.
Put the requirement into the work record
- Confirm which thickness you are planning. Find the member's required intumescent dry film thickness in the applicable design and project documents. Distinguish that requirement from the primer, topcoat and combined coating thickness. An overall gauge reading or a decorative coat allowance should not silently become the required intumescent layer.
- Check the actual calculation inputs. Read the selected product's volume-solids value and any qualifications in its current instructions. Ask the qualified applicator to confirm how the specified method uses that information. Do not assume that all products share the same solids fraction or that an unauthorized reduction leaves it unchanged.
- Divide the build into permitted coats. Establish each coat's target with the actual product limits and planned sequence. Keep the required total beside the coat plan. Dividing a total by two is arithmetic; it does not prove that two coats of that thickness are permitted or can be recoated on the desired date.
- Arrange meaningful wet-film checks. Have trained personnel use the appropriate instrument and actual measurement procedure. Record member, surface, coat, time, target and observations as required. A number without the coat or location cannot show where an adjustment is needed, and this outline does not teach comb placement or operation.
- Respond while the application is being controlled. Ask the lead to review a reading outside the planned range and follow the selected instructions. Keep the response and subsequent evidence with the coat record. Avoid adding an improvised heavy layer to compensate for a low reading without checking the permitted application route.
- Retain the final dry-film verification. Link wet checks to the later specified survey and keep discrepancies visible. Review what was actually measured and which layer the reported value represents. A calculated prediction can help the crew control work, but it does not replace the inspection or establish the member's hourly rating.
Common mistakes
- Using the required total as one coat's wet target.
- Treating an overall thickness as the intumescent layer.
- Calling the calculation a guaranteed inspection pass.
Checklist
Keep six thickness references
- Required intumescent dry thickness.
- Actual volume-solids input and qualifications.
- Permitted coat plan.
- Wet-film location and coat records.
- Responses to out-of-plan readings.
- Specified dry-film verification and findings.
Check your understanding
Does a calculated wet target prove that the finished intumescent thickness will pass inspection?
Show the answer
No. It supports application control under the stated assumptions. The actual coat restrictions and final dry-film survey still apply, including how primer and topcoat contributions are addressed.
