The Importance of Coating Thickness

Wood floor professionals spend years developing a skillset that relatively few possess. It is the ability to resurface a wood floor and artistically apply a protective finish that enhances both the appearance and the durability of the wood. While significant time is devoted to learning how to apply these coatings it is equally as important to understand why following application instructions is so important. As film-forming finishes are a steady favorite for wood floor refinishing projects, proper application and coating thickness become crucial factors (often ignored) for a successful outcome.

THICK VERSUS THIN COATS

Any discussion about coating thickness should begin with the understanding that not all finishes are designed to have the same filmbuild. While many water-based finishes specify a familiar 350 to 500 square feet per gallon coverage rate, not all fall into this category. For
example, roller applied hardwax oils have a coverage range of 800 to 1,230 square feet per gallon depending on brand and system.

Likewise, certain color-treating sealers, reactive products, and liquid moisture or vapor barrier systems are not intended to be applied at heavy film thickness. Applying products too thick or too thin can lead to improper curing, adhesion problems, or inconsistent results. For this reason, it is critical to review product labels and understand technical data sheets (TDS) for each coating system being used. Understanding why these specifications exist requires a look at how finishes are developed and tested by the manufacturer, which leads to the next part of the discussion.

When chemists develop water-based finishes, they test the limits of their formulations under a wide range of conditions. The coverage rate listed on the product TDS represents a target application range where the finish is expected to level properly, dry consistently, and develop the intended film properties. This recommended range reflects what the manufacturer has determined to be the optimal balance between user friendliness, appearance, and long-term performance. Deviation from these recommendations can lead to performance-related problems including poor leveling, drying issues, reduced chemical resistance, diminished wear-resistance, and an overall undesirable appearance to name a few.

Most finish manufacturers will suggest staying toward the lower end of the recommended coverage range for optimum results. Staying within the recommended coverage rates you will find that open working time is maximized, flow and leveling is better (even overcoming some application errors), slower penetration of harmful fluid spills, extended service life, and an overall film thickness that ensures coating performance as intended.

UNDERSTANDING PROPER FILM THICKNESS

It is important to note first, the difference between wet- and dry-film thickness. Wet-film thickness (WFT) is calculated at the time the coating is applied on the floor. Dry-film
thickness (DFT) is calculated after drying is completed and the remaining components in the finish have evaporated. The relationship between WFT and DFT is straightforward: the
dry-film is approximately the wet-film minus the portion lost to evaporation (this is commonly expressed as the solids content of the finish). For waterbased finishes with 30 to 35 percent solids content, roughly 65 to 70 percent of the applied coating will evaporate.
For example, a coat applied at 100 wet microns will yield 30 to 35 microns dry, while a 120-micron wet coat will result in 36 to 42 microns dry.

In the end, setting and use situation for any given floor must be logically and honestly evaluated when choosing the product, its application rate, and the proper application tools. For instance, with our products, in residential settings with light to moderate use, a good benchmark goal is 90 to 105 microns of dry film (three coats at 100 microns each). A typical
household with kids and pets should strive for a minimum of 108 to 126 microns of dry film (three coats at 120 microns each) and extreme use floors require 135 to 158 microns of dry film (three coats at 150 microns each). Achieving these film targets requires proper technique, selection of the appropriate tools, having sufficient product on hand, and uniform application from room to room.

The encouragement for floor finishers to adhere to recommended coverage rates while applying waterbased or other coatings is not a ploy or conspiracy to “sell more product.” Rather, it ensures that the intended performance of the coating is achieved, creating a successful outcome for all parties involved. When a homeowner receives a beautifully finished floor, they are happy with their chosen professional and investment. The professional benefits from using a product that performs reliably, and the manufacturer is happy to earn trust and repeat business. Maintaining this cycle of quality and trust is essential for the long-term health of our industry. May the odds be stacked in your favor for successful coating.

Damian Bush is technical director for Berger-Seidle America.

NOTE: 1 micron = 0.03937 mils; 1 mil = 25.4 microns
Why the difference matters: Specifications may be written in one unit but measured in the other. Using the wrong unit can lead to incorrect application, quality control failures, or non‑compliance.