While proper surface preparation is integral to the success of any painting project, this is especially true with concrete floors. Coating concrete floors can be a challenge, as there are specific factors that need to be taken into consideration to avoid systems failures.
Typically, concrete floors are “poured in place” and finished with a steel trowel to produce a smooth, dense surface. After the troweling process is complete, curing compounds are often applied in order to slow down the rate of water evaporation and improve the hydration and curing of the slab.
Both of these factors could cause problems for any potential coating system. This article presents information on how to deal with them.
Curing Compounds
Although curing compounds improve the concrete slab’s strength and hardness, as well as its physical and chemical properties, they are typically made from low-strength resins. These resins do not provide sufficient adhesion or film strength to function as either a primer or sealer.
If these compounds are not adequately removed, any coating applied over them will fail.
To accomplish this, the surface should be washed with a suitable detergent or emulsion cleaning solution and then thoroughly rinsed with clean water to remove any remnants of the curing compound.
One of the ways to test for any remaining compound is to apply a muriatic acid solution to various test areas of the surface. If the applied acid does not react with the concrete by bubbling, spitting, or foaming, that means there is residual curing compound remaining that requires additional cleaning.
Smooth Surface
The troweling process leaves a smooth surface on the concrete. If the surface is too smooth, there is no profile for an applied coating to adhere too. An ideally prepared concrete floor will have a texture similar to 100-grit sandpaper.
To accomplish a sufficient anchor pattern for the coating to adhere to, the surface should be abrasive blasted or acid etched.
Acid Etching
Surfaces should first be wetted down with clean water, followed by applying a recommended solution of muriatic acid (commercial quality, 30–38% hydrochloric acid). This is made by diluting one part of concentrated acid (see caution note) with two parts of water by volume.
Apply the solution at the rate of one gallon (4.5 L) per 100 square feet (9.3 m2), and scrub well while applying. Allow the solution to remain on the surface until it stops bubbling — usually, about 20 minutes — and then flush thoroughly with a large quantity of clean water.
If the surface does not dry uniformly within a few hours, not all of the acid has been removed. In this case, flush the area again, but use a weak solution of household ammonia and clean water. After the surface has dried thoroughly, painting may proceed.
Caution Note: When making acid, adequate handling precautions must be taken. Always refer to the material safety data sheet (MSDS) to become aware of relevant safety hazards. Any person handling, mixing, or applying acid solutions, should wear rubber gloves, aprons, boots, goggles, a face shield, and a proper breathing apparatus.
Do not mix acid with any other chemical, and when diluting, always add acid to water. Never add water to acid. Accidental splashes on the skin should be treated immediately by flushing with clean water. If burns are severe (or in all cases of eye contact), contact a physician immediately. Wash hands thoroughly before handling food.
Additional information is available in MPI’s Architectural Coatings Manual.
Environmental Conditions
As with all coating applications, environmental conditions should be considered. This should be done before and during application, as well as during drying.
Due to the density of concrete, the surface temperature must be monitored during cold or humid weather. The dew point should be calculated, as well.