Monthly Archives: January 2015

How to determine the minimum film thickness for an application

Customers frequently ask what our recommendation is for the minimum film thickness for a coating applied over concrete.

We see wide ranging specifications from 2-3 mils DFT (dry film thickness) on up. While the exact thickness will depend on a number of factors, including condition of the existing floor, desired life expectancy, type of traffic, budget, chemical exposure and finished appearance, there are a few essential facts that must be considered in trying to arrive at a minimum thickness.

Any floor coating manufacturer will specify that the floor must be clean and abraded, as a good mechanical “profile” is essential to long-term adhesion. In today’s environment, it is almost unheard of any more, at least on commercial-sized applications, to acid or chemically etch a floor; hence, mechanical prep is by far the most prevalent form of surface prep. Mechanical shot blasting is the most widely used form of prep and while the finished profile can widely vary, most material manufacturers want a minimum profile of 80 grit, with 60 grit more common for “thin film” coatings. Such a grit size corresponds to a profile range of 3.5-10mils. For more contaminated surfaces, or where cure and seal compounds must be removed, it is not uncommon for a 40 grit texture, equivalent to up to 16 mil maximum profile to be obtained.

With such profiles, any coating thickness not designed to cover the peaks of the profile is not doing its job, and, in fact, is leaving pinholes in the finished surface. For this reason, we recommend a two-coat, 20mil minimum thickness. Greater thicknesses are recommended on rougher floors, with a 60 mil self-leveling floor coating considered best for badly pitted floors. Anything less than 20 mils is going to be ineffective long term, especially in situations where the coating is expected to provide a protective barrier against chemicals, fuel oils and other contaminants. The advent of 100% solids epoxies and urethanes make achieving these minimum thicknesses in two coats much easier than in the case of solvent-based paints.