How do you measure paint and coating thickness?

When supplying paint to our customer, one question understandably crops up: how thick should this be applied? Rustbuster’s Chris Allen has the answer.

Over the years working in the coatings trade, I have heard many terms about paint thickness. People talk about ‘a good full coat’, ‘a heavy coat’ and ‘a thick coat’. But what thickness is actually required? And how can you make sure what you apply is within specification for the chosen coating?

How thick should the coating be?

All paints come with two data sheets. One is the Material Safety Data Sheet (MSDS) that features details relating to the make up of the product from a health and safety point of view.

But it’s the Technical Data Sheet (TDS) that features practical details relevant to using the product, including thickness. The two main thickness-related specifications are quite self-explanatory:

• Wet Film Thickness (WFT). This specifies how thick the product should be applied wet from the tin.
• Dry Film Thickness (DFT). This specifies how thick the product should be when dry.

There will almost always be a difference between the WFT and the DFT. That’s because most paints include a solvent that evaporates during drying.

The non-solvent component of paint is known as solids. If a paint product has an 80% volume of solids content, you can expect a 20% difference between the WFT and the DFT. This is because 20% of the product is solvent that will evaporate away as the paint dries.

As an example, here are the specifications for Rustbuster EM121 Epoxy Mastic:

• Recommended WFT between 115-445 microns (µm).
• Recommended DFT 100-400 microns (µm).

How do you measure coating thickness?

In most circumstances when applying paint to your vehicle, you only need to measure the Wet Film Thickness. If you accurately measure the WFT in line with the Technical Data Sheet, the DFT should take care of itself.

The simplest and cheapest way to measure this is with a WFT gauge, such as the one pictured here:

Let’s assume we decide to apply Rustbuster EM121 Epoxy Mastic to a WFT of 200 microns (µm). After applying the coating to a thickness we believe is in the right ballpark, we are ready to measure it.

First, choose the correct WFT gauge for the desired thickness. Here I have selected a comb that goes from 127 to 330 microns (µm).

Now place the WFT gauge onto the freshly painted coating as shown below.

In this case, the coating touched the tooth reading 178 microns (µm) and the first un-touched tooth reads 228 microns (µm). This provides us with a WFT of between 178-228 microns (µm) and is within specification tolerance. Now repeat the test a few times across the painted area to get an average.

Rustbuster supply plastic Wet Film Measuring Combs. A pack of 10 combs costs just £2.25 +VAT and really should be a necessary no-brainer accessory when purchasing coatings.

More elaborate equipment for measuring Dry Film Thickness is available, such as electromagnetic DFT gauges (below) and magnetic banana gauges. However, these are costly pieces of equipment and, let’s be honest, if you measure the WFT you can correct any issues there and then.

Need help with measuring paint thickness?

Rustbuster supplies a range of measuring tools that includes the following:

Holiday detectors. Detects pinholes and flaws in coatings.

Electronic DFT gauges.  DFT meters for accurate measuring of coatings on all metal substrates.

Salt testing kits. Measures the presence of salt and other contaminants on steel.

Testex gauges. Measures Testex Tape and determines the average maximum peak-to-valley height of a blasted profile.

Dew point meters. Accurately calculates the current dew point at the point of coating application.

If you need advice on selecting or using these or other types of testing equipment then please get in touch: email sales@rust.co.uk or phone 01775 761 222.