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Guides / Application guide

Film Thickness Measurement

Why film thickness matters, how to measure wet and dry film thickness, and what to do when readings are out of range.

Application guide · 2 min read

A wet-film thickness gauge resting on a completed Marindec quality-control record
Wet-film gauge on a batch QC record — how the figures on our product pages are measured

Why film thickness matters

Applying a coating within its target DFT range is as important as correct surface preparation. Coating outside the target range causes predictable failures:

Failure typeCauseResult
Pinhole rustingDFT too low — substrate profile peaks through the filmLocalised electrochemical corrosion at exposed peaks
BritlingPrimer applied below target DFTPoor intercoat adhesion; topcoat flexibility lost
Sagging / runningDFT too high — viscosity and gravity conflictInconsistent film, uneven protection
Stress crackingEpoxy applied above target DFTResidual stress exceeds adhesion strength — cohesive fracture
WrinklingAlkyd topcoat too thickTopcoat skins before undercoat — relative motion causes surface distortion

A 0.5mm variance in wet film thickness can change the final DFT by 32–50% of the target value.

Measuring Wet Film Thickness (WFT)

Measure WFT immediately after application, before any drying occurs.

ToolToleranceMethod
Comb gauge±5%Press into wet film; highest notch that contacts the film indicates WFT
Wheel gauge±2.5% (+1μm)Roll across wet film; eccentric edge marks WFT at touch-dry point

Notes:

  • Clean gauges with solvent immediately after each reading
  • Wheel gauge: inspect eccentric surface for chips before use — chips cause skip marks and inaccurate readings
  • Apply at 3 or more random points per panel/area to capture thickness variation

Measuring Dry Film Thickness (DFT)

Use an ELCOMETER (or equivalent electromagnetic gauge) on ferrous substrates after full cure.

  • Calibrate against manufacturer shims before use — confirm calibration at start and end of each measurement session
  • Take readings at a minimum of 3 random points per surface area
  • Record the mean and range; both minimum and maximum readings matter

WFT to DFT calculation

To estimate DFT from a WFT reading:

```

DFT = WFT × (NVC% ÷ 100)

```

NVC% = Non-Volatile Content by volume — available from the product TDS.

Example: Product with 50% NVC applied at 100μm WFT → estimated DFT = 50μm

To work backwards (target DFT → required WFT):

```

WFT required = Target DFT ÷ (NVC% ÷ 100)

```

Example: Target DFT 60μm, NVC 50% → apply at WFT = 120μm

If film thickness is out of range

SituationAction
DFT below minimumApply an additional coat once the existing film has cured to the minimum recoat window
DFT above maximumDo not apply additional coats; allow full cure; consult TDS or contact Marindec technical support

*For product-specific DFT ranges, recoat windows, and dilution rates, refer to the Technical Data Sheet (TDS) for each Marindec product.*

Download the PDF guide

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