Using A Wet Film Mil Gauge

By Jef McCurdy

Most of us know the manufacturers recommended mil thickness for specific coatings we may be using. But, do you actually know how to make sure you are on spec? A silicone manufacturer, for example, may require 24 dry mils for a 10 year warranty. If this is a 96% solids silicone, you will only need to apply 25 wet mils to cure to 24. If the silicone is solvent based and 80% solids, you will need to apply at 29 wet mils.

Manufacturers spec sheets will give you theoretical coverage rates, but do not account for age and other conditions of the substrate. Old, weathered TPO may require more product for the same mil thickness than newer metal. This is where your wet film mil gauge becomes a very useful tool.

The good news is, they are very cheap and easy to use. Common gauges are shaped either like a credit card or a hexagon and consist of various depth notches on each side. Many paint stores and coatings manufacturers offer them for free. Each notch represents a different mil thickness (each mil is 1/1000″). To use, set the gauge gently into the coating on a flat section of the project.

Wet mil gauge in silicone over Ecodur base.

Now, lift straight up without dragging and look to see which notches have product on them. For example, let’s say that your 24 mil notch has some silicone on it, but 26 is dry. Your true reading is going to be about 25 mils.

In this example, we were looking for 34 wet mils. A little over, but close.

So, why does any of this matter? For NDL’s and other manufacturers warranties, you must achieve the minimal thickness or they may reject the protection. Alternatively, applying too thickly will cost your project margins and potentially cause product shortages. Being a few mils over will likely not cause a problem, but your coverage rates will be way off if you apply with excessive thickness. Some products also will not cure properly if applied too thick.

Measuring your mils during application takes only a few moments and can ensure that you and your crew are staying on track.

A quick check with each new bucket can ensure proper coverage and consistent results.

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