The solvent in a coating will immediately start to evaporate after spraying. In order to achieve a common method of reading the coating thickness, a time frame will need to be established. Typically, one might measure the thickness 5 to 10 seconds after spraying. If another operator measures the thickness after 20 seconds, the results would be different even if the initial thickness was identical.
Reading with Clear Coats
A clear coating on a WFT gauge would be very difficult to read. The most common method of reading clear coats is to use the gauge as a stamp on a piece of absorbent (non-gloss) paper. Many companies use the stamp method as a way of documenting the WFT.
Creating Surface Defects
After using a WFT gauge to check the film thickness, the material may not flow to hide the area where the gauge 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 part. The sample then may be checked for WFT and DFT (after curing).
The Simplicity of Coating Coveage Claims
Now that we've discussed the simple and proper method of using a Wet Film Thickness gauge, let's now look at how some simple mathmatics can help you determine whether the coating that is being marketed to you has any credibilty. Knowing a product's specified wet and dry film thickness; and understand that what you see on the label may be different 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, I 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.
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 huge for assuring your customers expectations in advance of starting a job. It might be the difference between getting the job and the job going to your competitor!