January 5, 2021

Wet Film Thickness: The Standard for Applying Coatings and Estimating Accurately

Applying coatings to manufacturer recommended specifications is simple if you have the right tools and if you understand some general coating standards. In this article, we're going to look at the "wet film thickness gage" as our standard tool for accomplishing manufacturer recommended wet/dry film thickness specs. We're also going to look at some common sense numbers so that you can quickly judge if a manufacturer's claim makes sense or not based on some simple arithmetic.

First, let's talk about the wet film thickness gage. A wet film thickness gage is designed to give the coating's applicator immediate feedback as to the film build applied to a given surface. In most cases, measuring the dry film thickness (DFT) provides little information as it is usually measured a considerable amount of time after the application. Many things could have influenced the DFT: operator fatigue, ambient air temperature, coating temperature, etc.

There are several types of WFT gages available. The most common being the notch gage (see figure 1). Others types including the eccentric disk, the rolling notch gage and the 6 sided gages are available from specialty vendors.

Figure 1

There are several issues that must be addressed when using a WFT gage.

  • Technique
  • Timing
  • Reading with clear coats
  • Creating surface defects


When placing gage must be placed 90 degrees to the surface. The operator also needs to be aware of variation of the surface that may influence the reading. If the surface is not perfectly flat, one direction may give a more accurate reading than another. So for example, on a piece of wood, the gage should be used perpendicular to the grain of the surface to account for the highest levels within the grain. To use the WFT gage, place the gage directly on the wet finished surface (see figure 2) and as described above. The notches will indicate the measured film thickness. For example, if the 1 and 2 mil notches are wet and the 3 and 4 notches are dry, then the measured thickness is between 2 and 3 mils (.002 to .003 inches).


The solvent (including water) in a coating will immediately start to evaporate after spraying. In order to achieve a standard method of reading the coating thickness, a time frame will need to be established. Typically, one might measure the thickness 30 - 60 seconds after spraying. If another operator measures the thickness after 5 minutes, the results may be different even if the initial thickness was identical. Water-based coatings, like those from Enviroguard, don’t have this issue to the same degree because the dry times are longer than that of solvent based coatings. However, measuring WFT before the drying process begins is still important. As a general rule, measuring the WFT of water-based coatings should be done within 5 to 10 minutes of application for accuracy.

Reading with Clear Coats

A clear coating on a WFT gage can be very difficult to read. The most common method of reading clear coats is to use the gage as a stamp on a piece of absorbent (non-gloss) paper. Many companies use the stamp method as a way of documenting the WFT. Fortunately, products like ClearGuard and VaporLock Clear, apply white and dry clear, making them much easier to read in the field.

Creating Surface Defects

After using a WFT gage to check the film thickness, the material may not flow to hide the area where the gage was used. If this creates an undesirable defect, place a small sample of the material in line with the operators normal spray path. This sample should be sprayed along with the surface. If necessary, the sample then may be checked for DFT (after curing).

The Simplicity of Coating Coverage Claims

Now that we've discussed the simple and proper method of using a Wet Film Thickness gage, let's now look at how some simple mathematics can help you determine whether the coating that is being marketed to you is reasonable in its coverage specifications. Knowing a product's specified wet and dry film thickness; and understanding that what you see on the label may differ from a product's warranted coverage wet and dry film thickness coverage rates, will save you liability and it will ensure that you are estimating for the job that your customer believes they are buying.

The standard number by which all coatings are measured is 1600. Let's use a hypothetical coating example where we know that for every 1 wet mil of coating applied, it will dry down to half that thickness 0.5 dry mils. If a manufacturer is claiming that a coating will cover 400 square feet per gallon, then you'll divide 400 into 1600. This tells you that the manufacturer is recommending 4 wet mils (2 dry mils per our example) of coating on the surface to accomplish its labeling claims. This is where performance claims, coverage claims and aesthetics often diverge. In many cases, a manufacturer might show 400 square feet per gallon on its label for the purpose of marketing its coverage claims. In other words, the product can cover up to 400 feet per gallon without gaps in coverage that would otherwise leave the surface unprotected. However, the performance claim on a product that may be acceptable for product warranty coverage may be closer to 20 wet mils (10 dry mils per our example) for the same product. If you divide 20 wet mils into 1600, then you'll quickly see that in order to achieve warranted coverage, the product should be applied at 80 square feet per gallon. As you can see, this makes a huge difference in properly estimating a job and it makes a huge difference in the way that a product's warranty (if applicable) is understood by both you and your customer.

Finally, from years of working with contractors and their customers, we can tell you that aesthetics and customer expectation play a huge role in a customer's satisfaction when you finish a job. A label may say 400 square feet per gallon and a manufacturer's warranty statement may indicate 80 square feet per gallon, but if the customer doesn't think it looks as they expected it to look, then there's another potential problem that could cost you profit. We recommend carrying samples with you to show customers approximately what a product will look like once it's applied to a surface (wood, masonry, etc) and that you show them samples according to the product specification and related coverage claims and warranty claims that are stated by the manufacturer (see Figure 3). In this picture, you can clearly see two different application specs since we used a black marker to visually indicate the opacity of the coating for aesthetic values like stain-hiding capacity. This is a simple, inexpensive and great reference tool to use with customers.

Figure 3

Even when working with clear coatings, it can be important to provide this same level of visual assistance for your customer. For example, in the Figure 4, you can see that while the clear coat creates a deeper or richer look on wood, it doesn't leave behind any shine. Since we deal in mostly residential and commercial structural projects this can be important for assuring your customer's expectations in advance of starting a job. It might be the difference between getting the job and the job going to your competitor!

Figure 4

We hope that this article has been helpful in articulating both how to properly control the quality of your coating applications and how to determine that you're getting the maximum value for your coating purchase. If you have questions on this or on other technical topics, please email or call us any time.

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