Measuring wet film thickness

 It is important to have a good “read” on the actual wet thickness of your coating application.     

 Using the Gauge

When you spray finishes, the wet mil gauge helps you to get consistently good results. It is about the size and shape of a credit card[1]. It has a number of cutouts on each side that become progressively shorter by 1mil increments (about the thickness of a sheet of paper). On your gauge you will see the numbers 1-6 and 7-12 on the sides, and 14-30 and 35-80 at the top and bottom. Each of these tabs is labeled to indicate how far they are from the surface you set the gauge on.

 To use the gauge, you first spray a flat surface and immediately set the gauge on the wet finish (this is easier to do on horizontal surfaces). You then lift the gauge and count the impressions it left in the finish.

 Reading the Gauge

When you lift the gauge, you will see a number of impressions in the wet finish (they will disappear). Count the number of marks in the finish, not including the ends where the gauge feet contacted the surface. The number of impressions will tell you how many of the mil tabs touched the finish. This in turn tells you how thick you sprayed the finish.

 After counting the marks, use a clean cloth to wipe the gauge clean. If you allow finish to build up on the measuring tabs you will get incorrect readings from the gauge. After using the wet mil gauge for a while, you will get an “eye” for how the finish looks when sprayed at the recommended thickness.


Most finishes used on furniture are sprayed in the 1-5 mil range, though some will be applied thicker. If you have thinned the finish to adjust its viscosity, you will have to spray thinner coats to avoid runs and sags.

 Be sure to consult the data sheet or your Columbia Industrial Supplies sales representative for the recommended mil thickness of your finish. Your finish may also have a maximum number of coats you should apply.

[1] If you don’t have a gauge, we would be happy to supply one for you.