The fundamental difference between wet film thickness and dry film thickness is the state of the coating when the measurement is taken: wet film thickness (WFT) is the thickness immediately after application, while dry film thickness (DFT) is the thickness after the coating has dried or cured and volatile components have evaporated.
Understanding Wet Film Thickness (WFT)
Wet film thickness refers to the thickness of a coating layer immediately after it has been applied to a surface, but before any significant drying, curing, or evaporation of solvents or water has occurred.
- Measurement Timing: Taken when the coating is still in its liquid or semi-liquid state.
- What it Includes: It includes both the solid components (pigments, binders, additives) and the volatile components (solvents, water).
- Purpose: Often measured during application to ensure the correct amount of coating is being applied according to specifications.
Understanding Dry Film Thickness (DFT)
Dry film thickness refers to the thickness of the coating layer after it has fully dried or cured. At this stage, the volatile components (solvents or water) have evaporated, leaving behind only the solid material of the coating.
- Measurement Timing: Taken after the coating has dried, cured, and all volatile components have evaporated.
- What it Includes: It represents the thickness of the solid binder, pigments, and additives that form the protective and decorative film.
- Purpose: DFT is the final thickness that determines the long-term performance characteristics of the coating, such as corrosion protection, durability, and abrasion resistance. Coating specifications are typically given in terms of required DFT.
The Relationship and Calculation
Because the volatile components in the wet coating evaporate during the drying/curing process, the wet film thickness will always be greater than or equal to the dry film thickness. The difference between WFT and DFT is the thickness lost due to evaporation.
The dry film thickness can be calculated using the 'solids to liquids' ratio, also known as the volume solids content. This percentage represents the proportion of the wet coating volume that will remain as solid film after drying.
For example: if the paint has a volume solids value of 80% and the wet film thickness is 200 µm, the dry film thickness will be 160 µm (200 µm x 80% = 160 µm).
The formula is:
DFT = WFT × Volume Solids (%)
Conversely, you can estimate the required WFT to achieve a target DFT:
Required WFT = Target DFT / Volume Solids (%)
Key Differences Summarized
Here is a table highlighting the main differences between wet and dry film thickness:
| Feature | Wet Film Thickness (WFT) | Dry Film Thickness (DFT) |
|---|---|---|
| Measurement State | Liquid or semi-liquid (wet) | Solid (dry or cured) |
| Measurement Time | Immediately after application | After drying or curing is complete |
| Components | Solids + Volatile (Solvents/Water) | Solids only |
| Thickness | Always greater than or equal to DFT | Always less than or equal to WFT |
| Purpose | Application control, ensuring proper spread | Final film performance, adherence to specification |
| Calculation Relation | Used to calculate DFT (with Volume Solids) | Calculated from WFT (with Volume Solids) |
Why Measure Both?
Measuring WFT helps applicators control the amount of coating applied during the job, ensuring they are on track to meet the final DFT requirements without applying too much (waste, potential defects) or too little (failure to meet specification, inadequate protection). Measuring DFT confirms that the applied coating meets the required thickness for optimal performance and longevity.
