Mil thickness measures the thickness of an applied coating, with one mil equal to one thousandth of an inch. Contractors use it to describe both how much material was applied and how thick the finished coating ends up being.
Wet Film Thickness vs. Dry Film Thickness
Wet film thickness is measured immediately after application, while the coating is still liquid, and is checked with a small notched gauge dragged through the wet material. Dry film thickness is the final cured thickness once solvents or water have evaporated and the resin has fully reacted. For 100% solids epoxy, wet and dry thickness are close to the same, but for solvent or water based coatings, the dry thickness can be noticeably less than what went on wet, since a portion of the material evaporates during cure.
Typical Mil Thickness Ranges
Thin, roll-on coatings like DIY garage kits often build to somewhere in the range of a few mils per coat, while full broadcast flake systems and heavier commercial builds can reach well beyond that once primer, base coat, and topcoat layers are added together. There is no single correct number for every job. The right thickness depends on the traffic, chemical exposure, and system design, so the manufacturer’s technical data sheet should always be the final word.
Practical Tips for Managing Mil Thickness
Use a wet film gauge while applying each coat to catch thin or heavy spots before the material cures, since it is much harder to fix thickness problems after the fact. Keep in mind that spreading material too thin to stretch coverage is one of the most common causes of premature coating failure, so it is worth using enough material to hit the specified build rather than cutting it close.
