Avoiding Common Failures: Runs, Pinholes, and Under-Thickness in Field Conditions
Most intumescent failures are not dramatic.
They are quiet, repetitive, and familiar.
Runs down a web that were left to cure.
Pinholes dismissed as cosmetic.
Areas that look coated but measure thin when tested properly.
These defects rarely result from poor products. They arise from loss of control — of environment, technique, sequencing, and inspection. In field conditions, intumescent coatings are unforgiving. They respond exactly to how they are applied, not how they were intended to be applied.
Fire resistance is achieved through discipline, not optimism.
Why Minor Defects Become Major Fire Risks
Intumescent coatings rely on uniform film build and continuous adhesion to perform as tested. Localised defects interrupt that continuity. In fire conditions, heat does not distribute evenly; it exploits weaknesses.
A run introduces variable thickness and internal stress.
A pinhole becomes a pathway for heat and gas.
An under-thick area reaches critical steel temperature early.
Each defect reduces predictability. Enough of them, and the system no longer behaves like the test evidence assumed.
The danger lies in scale. One defect may be tolerable. Repeated defects across a structure are not.
Runs — When Gravity Wins
Runs occur when the coating exceeds its ability to hold itself in place before curing. This is not merely a visual issue. Runs create uneven film build, weak internal structure, and areas prone to cracking as the coating dries and later expands.
Primary Causes of Runs
Runs are most often caused by:
• Excessive Wet Film Thickness in a single pass
• Low ambient or steel temperatures slowing curing
• Incorrect spray pressure or technique
• Poor viscosity control or over-thinning
• Attempting to accelerate programme by heavy loading
Gravity will always win against uncured material.
Why Runs Matter in Fire Conditions
Runs concentrate material in one area and starve another. The thicker section may crack or delaminate; the thinner adjacent section heats faster. Expansion becomes uneven, placing stress on the bond to the primer.
Runs are not self-correcting. Once cured, they lock in inconsistency.
Pinholes — Small Voids, Real Consequences
Pinholes are often dismissed because they are small. That dismissal is a mistake.
Pinholes represent trapped air, solvent release, or surface contamination breaking through the film during curing. Each pinhole is a discontinuity in insulation and a potential failure initiation point.
Common Causes of Pinholing
Pinholes typically arise from:
• Contaminated or poorly prepared substrates
• Application over porous or improperly sealed primers
• Excessive spray pressure introducing air
• Rapid solvent evaporation due to high temperature or airflow
• Inadequate flash-off time between coats
They indicate instability in the application process.
Why Pinholes Matter
Under fire exposure, pinholes become heat concentrators. They compromise insulation locally and can propagate cracking during expansion. In aggregate, they erode the assumed resistance period.
Pinholes are not cosmetic defects. They are performance defects.
Under-Thickness — The Most Common Hidden Failure
Under-thickness is the failure most likely to pass visual inspection and the most likely to undermine fire performance.
Intumescent coatings do not protect by coverage alone. They protect by measured film build. Areas that appear coated may be significantly below the required DFT.
How Under-Thickness Occurs
Typical causes include:
• Inaccurate WFT calculation
• Over-reliance on visual judgement
• Irregular steel profiles receiving uneven coverage
• Loss of material to overspray
• Failure to build up required coats
• Measuring selectively rather than systematically
Under-thickness is rarely accidental. It is usually procedural.
Why Under-Thickness Is Critical
Fire resistance time is directly linked to DFT. A reduction of even a small margin can materially reduce the protection period. Steel reaches critical temperature faster, regardless of how good the surrounding areas look.
Fire does not average thickness. It finds the thinnest point.
Field Conditions — The Multiplier of Risk
Site conditions amplify all three failure modes.
Incomplete building envelopes, fluctuating temperatures, wind, dust, moisture, and programme pressure all increase the likelihood of defects. Field application demands more control, not less.
Without environmental monitoring, even skilled applicators are working blind.
Application Discipline — Control Before Correction
Most defects cannot be corrected after curing without rework. Prevention is therefore the only viable strategy.
Effective control includes:
• Applying within manufacturer-specified environmental limits
• Controlling WFT rather than chasing DFT at the end
• Applying multiple controlled coats rather than heavy passes
• Maintaining correct spray distance and angle
• Allowing proper flash-off and curing times
• Protecting work between coats
Correction should be the exception, not the plan.
Inspection and Measurement — Where Reality Is Confirmed
Visual inspection alone is insufficient.
Effective inspection requires:
• Systematic DFT measurement across all steel faces
• Particular attention to edges, flanges, and web junctions
• Identification and remediation of low readings
• Documentation tied to specific members
Spot checks confirm nothing. Coverage must be representative.
Repairing Defects — Doing Less, Properly
When defects are identified, repair must be proportionate and controlled.
• Runs should be carefully dressed and recoated, not buried
• Pinholes require investigation of root cause before repair
• Under-thickness demands measured build-up, not guesswork
Over-application creates new defects. Correction must restore uniformity, not overwhelm it.
Documentation and Evidence
Modern compliance requires that defects and corrections are recorded.
This includes:
• Initial inspection records
• Identified defects and locations
• Remedial actions taken
• Post-remedial DFT readings
• Photographic evidence
A corrected defect that is undocumented may as well not exist.
Conclusion — Fire Resistance Is Intolerant of Casual Errors
Runs, pinholes, and under-thickness are often treated as finishing issues. They are not. They are structural weaknesses in a life-safety system.
Intumescent coatings perform exactly as they are applied. They do not compensate for haste, assumption, or approximation.
Avoiding failure in field conditions requires restraint, measurement, and patience — the willingness to let chemistry and physics dictate pace rather than programme pressure.
When fire arrives, it will not judge intent.
It will test the weakest point.