Understanding Coating Thickness Measurements

When undertaking a project with a high performance coating, you need to think about how you will measure the coating thickness to ensure it is applied to specification. By monitoring and recording your coating thickness measurements you can ensure that you get a quality finish, that will perform as intended and as an added bonus you won’t unnecessarily waste material and money.

The thickness of a coating is often key in ensuring your coating performs the way it should both at the time of application and when in service. Generally the thicker it is, the more protection it will offer to your substrate but this isn’t always the case, sometimes over applying a coating is just as bad as under applying it. Coating thickness may be important for waterproofing properties, corrosion resistance, adhesion performance on tricky surfaces, aesthetic effects, and more but in most cases it is used with paint for steel.

There are two methods you can try to test the coating thickness:

Coating thickness meters

Coating Thickness Meters are used when a coating has already been applied and is cured, they use probes which when against the surface, give you a reading. A coating thickness gauge is often used at various stages through a project to check paint thickness in existing coatings on a surface to ensure they are suitable to over paint, checking your progress throughout a project to ensure each coat applied is up to standard and finally at the end of a project to check that all areas have received the DFT (Dry Film Thickness) they should have to meet specification. If you want an easy tool to use, we suggest the Coating Thickness Meter (C5007).  This meter is ideal for measuring all coatings on any metallic substrates. The magnetic induction probes using eddy current principles mean you can check that the correct coating thickness has been applied to your surface. This tool is one of the most advanced on the market, mean that you can feel assured you can make the right decisions with your coatings.

Wet film measurements

Another way to measure coatings is to use wet film measurements. This method measures the thickness of the coating, whilst it is still wet – WFT (Wet Film Thickness). We think Tricomb Plastic Wet Film Gauges (W2008) are ideal for this, as you can quickly and cheaply test the wet film thickness and even leave them to dry as a way of recording the coating thickness. These wet film gauges are disposable and have metric values on the front side of the gauge, and imperial values on the back for your measurements. The advantage of wet film gauges over dry film thickness meters is that they allow the applicator of the coating to adjust the application immediately as necessary to either increase the thickness they are applying at or decrease the thickness they are applying to the substrate to ensure correct application.

Once you have decided which method you’d like to use to monitor your application, or indeed if you decide to use both to be thorough, you need to make sure you understand the difference between Wet Film Thickness and Dry Film Thickness. Although the description makes them fairly self-explanatory it can be easy to work to the wrong figure from a specification or data sheet when on site or under pressure to complete on time especially when they are abbreviated to WFT and DFT.

The Wet Film Thickness figure given on a specification relates to the thickness of the coating when it has just been applied and is still wet, as soon as the coating begins its drying process solvents and volatiles will be leaving the coating and its thickness will reduce and once cured it’s Dry Film Thickness can be read. How much the thickness reduces will depend on the Volume Solids of the coating, the higher the volume solids of the product the less things there are to escape from the coating so the less the thickness will reduce. For example, if a coating specifies you apply it at 500 microns WFT (Wet Film Thickness) and the volume solids of the product are 50% then once dry the DFT (Dry Film Thickness) reading will be 250 microns. Some coatings are 100% volume solids which means nothing thickness will be lost between WFT and DFT.

Finally, here in the UK paint is usually measured in microns (often displayed as µm). 1µm (micron) = 1/1000mm so when you get up to 1000µm your coating will be 1mm thick. In the USA they tend to use Mil or Thou as their form of measurement, 1 Mil = 1/1000th Inch. 1 Mil / Thou = 25.4 Microns (µm).

If you’re not sure which method to use, or how to read the results. Just ask the experts at Rawlins Paints!

Posted: 15th September 2015 | Updated: 10th October 2024