Below are excerpts from an article written in September 15, 2020 issue of Concrete Decor magazine by Chris Sullivan:
Ever wonder where 21 square feet per bag or 300 square feet per gallon comes from? Coverage or application spread rates aren’t just a made-up number or educated guess. Determining coverage rates is always based on a mathematical constant known as the Theoretical Coverage Rate of a Coating. This in conjunction with surface conditions is how the rates are calculated.
According to www.powerwraplp.com, “The theoretical spreading rate or coverage of a coating is a function of the volume solids. The volume solids is the percentage of the coating consisting of binder and pigments which remain on the substrate, forming the final film after the volatile
Using this constant and the solids percentage of the product allows us to calculate the coverage rate of any material when applied to a smooth nonporous surface.components or solvents evaporate.”
A less technical explanation translates into how much of the actual solids of the applied product will remain and how far it will go once the volatile liquid portion evaporates. The mathematical part is based on the constant that 1 gallon of any 100% solids material will cover 1,604 square feet when applied at a dry thickness of 1 mil (1/1000th inch).
One U.S. gallon of sealer with 25% solids by volume will cover 267 square feet at a dry film thickness of 1.5 mil.
1,604 sq. ft. / 1.5 mil thick X 0.25 solids = 267 square feet
Most manufacturers will provide coverage ranges to take into consideration variations in surface smoothness and porosity. The smoother a surface the greater the coverage rate will be. The more porous a surface the lower the coverage rate will be. Understanding how coverage rates are established lets us better understand common decorative materials’ rate of application.
Decorative overlays are treated like a 100% solids coating when it comes to coverage rate. In the case of microtoppings, the application is thin, with most ranging from 1/16 inch to 1/8 inch.
Coverage ranges from 200 to 400 square feet per 50-pound bag. The variation in coverage comes from the final thickness. It is usually determined by the top size aggregate (sand) in the mix.
Single-component acrylic sealers usually top out at 30% solids. These are applied between 1 to 3 mils thick. Therefore, after working the math, you end up with 481 to 160 square feet depending on thickness.
Looking at a high-performance coating, like a 100% solids epoxy applied 5 mils thick, the theoretical coverage rate is 320 square feet. This is right between the 250 and 350 square feet most manufacturers state in their tech data sheets.
The big variable in all these applications is the surface profile and porosity. As stated earlier, the theoretical values are based on a smooth surface. Any roughness of the surface negatively impacts coverage rate.