Measuring finish thickness

How can you measure the mils of your applied finish? November 7, 2001

Various finishes that I use call for coats that are only 'so-many' mils thick. Is there an easy way to tell how thick the coats I am applying are? How thick would a garden-variety average coat of finish be?

Forum Responses
If you ask your finish supplier, they should have a wet mil gauge (hopefully free). After spraying a coat of finish onto some scrap material, you place the gauge, which has teeth that are above the surface in varying thicknesses, and see the thickest one to touch the top of the finish. That will tell you how thick your finish is.

The lacquer I use is supposed to be 4-5mils wet. But this will vary from one brand to another.

As mentioned above, any supplier should give you a free mil gauge.

Basically, if you are using normal wood coatings at 26 to 33 sec. zahn #2--most pre-cats, CVs and lacquers fall in to this range--the below will hold true.

1. A light coating with no dry spray will measure 3 wet mils.
2. A coat almost ready to run is 5 wet mils.
3. A coat with a sag, 6+ (unless you are a good sprayer, and then you can hang 6 without a sag).

Until you get the touch or if you want to spot check your finisher, the wet mil gauge is the answer.

It is true that viscosity is one good measure to check the relative thickness of the film, but more important is the solid content. Most pre-cats have a weight solid content in the upper 20s, whereas conversion varnishes have a typical solid content in the low 40s. Check your wet film thickness with the gauge, but multiply that wet film number by the solid content. This is a good measure for what your overall dry film thickness will be based on the total number of coats applied.

Here are 2 important points to remember. Lacquers will melt-in and partially dissolve the previous coat, thus forming one coat bonded together through chemical adhesion. If you are going to build, I always suggest you build with your topcoat and never with any type of sealer or pigmented undercoater. Multiple coats of an inferior sealer can be more prone to cracking because they are not as flexible when compared to topcoats.