Acrylic conformal coatings, such as MG Chemicals 419C and Humiseal 1B31 are a very popular conformal coating choice.  They are used primarily for moisture protection on printed circuit boards.  Often times, they are the cheapest conformal coating to apply because of their easy to use nature.  As a result of this, people try to apply acrylic conformal coating as a DIY project.  An issue that arises frequently is an improper thickness of coating, resulting in lackluster protection.

So what is a good thickness for acrylics?  According the IPC-610, type AR conformal coatings should be between .002” and .005”.  The MIL-I-46058C calls for .002” +/- .001”.  It seems that the sweet spot for acrylics is about .002” to .003”.  Meeting this thickness will ensure that you are within spec for two of the major specifications.

These standards call for these thicknesses so that the conformal coating reaches its optimum protection capabilities called out in the product spec sheet.  If thicknesses are not met, it could lead to the failure of the coating and ultimately the printed circuit board it is trying to protect.  Specifically for acrylics, the thickness helps prevent moisture ingress on the PCB.

Parylene Thickness Whitepaper