Measuring accurately matters in woodworking and finishing. There are many things that need to be measured accurately for optimal finishing. Last month’s Why Finish Measuring Accuracy Matters—Part 3 covered measuring wet film thickness. This month dry film thickness is covered.
Measuring accuracy matters a great deal with dry film thickness. Dry film thickness is vital to the performance of coatings. Each coating has a range from a minimum to a maximum dry film thickness that is optimum for performance. Knowing the actual dry film thickness verifies whether or not there is the optimum amount of dry film to achieve the performance properties desired.
Many of the finishes properties are directly related to achieving the correct dry film thickness. Too much finish and they may prematurely crack, entrap air, appear a higher sheen, etcetera. Too little finish and it may not protect the substrate as well, appear a lower sheen, etcetera.
A case where the dry film thickness led to quick failure involved a conference table top that the finish cracked within in a couple of months. The dry film thickness was tested with an ultrasonic film thickness device. It was determined the conference top had 6 to 12 dry mils of catalyzed lacquer applied. 6 mils around the edges up to 12 mils in the middle of the top. The recommended maximum dry film thickness for this coating was 6 dry mils. When analyzing the failure, it was determined that several coats of finish had been applied due to dust landing in the wet finish at each coat. They had tried to sand thoroughly between coats, however, they did not have a non-destructive way to test the film thickness. The top had been installed at the customer’s location and had to be picked back up and refinished while the customer waited.
Multiple ways of measuring the dry film thickness have been used over the years. Some methods destroy the finish and others are nondestructive. We will look at a few ways to measure the dry film thickness and focus on one of the best non-destructive methods.
An older and destructive method is obtaining a “chip” of the finish and using calipers to measure the thickness. There are a couple of issues here, the finish must be removed from the part causing a defect where the film was removed and getting a good “chip” can be difficult if the coating has good adhesion.
Another destructive method is to cut the finished piece in half and measure the film thickness using a microscope with distance measuring capabilities. The price of these microscopes has reduced significantly over the years and can provide good accuracy.
A different older way is to spray a test panel with part of the panel covered with a metal panel and finish according to the finishing process. Use a magnetic pull-off gauge, commonly known as a banana gauge because of its basic shape. While this gauge was developed to test metal finishes most film forming finishes can be applied to metal for testing.
Another way is to use a device to make a “V” cut through the finish and use a microscope with distance measuring capabilities to measure the layers of finish looking down. The specific device to make the “V” cut has a known angle of cut so the measurement looking from top down can be calculated to know the thickness.
Fortunately, advances in measuring dry film thickness have increased significantly over the years and we now have nondestructive ultrasonic measuring devices. These devices send ultrasonic waves through a substrate and measure how the waves bounce back. This allows the devices to accurately measure various thicknesses of coatings on a substrate, even a non-metallic substrate such as wood.
Make sure to use an ultrasonic gauge and probe recommended for wood. It is also important to check calibration, with the specific standards, to verify it is reading correctly. Simply apply the gel to an area to test and press the probe down. In a second or two the device can provide an accurate reading. Be sure to avoid areas of grain, as that can lead to false readings. Most devices will measure to within 0.1 to 0.01 dry mills and cost between $1000 and $1500.
Knowing the dry film thickness can verify whether or not the finish will perform optimally. Always consult with your finish supplier to know the specified range of dry film thickness for the coatings you are using.