Concrete Floor Surface Preparation Tips

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. In order to slow down the rate of water evaporation and improve hydration and curing of the slab, curing compounds are often applied after the troweling process is complete.

Both of these factors could cause problems for any potential coating system. Below you will find information on how to deal with them.

Curing Compounds 
Although curing compounds improve the strength and hardness, as well as the physical and chemical properties of the concrete slab, they are typically made from low strength resins that 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. 

In order to accomplish this, the surface should be washed with a suitable detergent or an 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. The acid solution cannot penetrate through the curing compounds, so where there is no bubbling, spitting or foaming where the solution is applied, the surface is not sufficiently clean for painting and should be re-cleaned.

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.  

In order to accomplish a sufficient anchor pattern for the coating to adhere to the surface should be abrasive blasted or acid etched.

Acid etching
In addition to providing a better surface profile or anchor pattern for adhesion, etching the surface will also lower the surface pH levels of the concrete. Alkaline (basic) surfaces like new concrete can impede the adhesion of coatings that are not designed to perform in high alkaline environments, etching the surface with an acid prior to coating will not only provide the slightly rough surface needed for the paint to grip, but also a more neutral pH level that will result in a more consistent and durable bond between the coating and the substrate.

Surfaces shall first be wetted down with clean water, then a solution of muriatic acid (commercial quality – 30 to 38% Hydrochloric acid) made by diluting one part of concentrated acid (See Caution) with two parts of water by volume, is recommended. Apply at the rate of one gallon (4.5 liters) of the solution to each 100 square feet, and scrub well while applying. Allow the solution to remain on the surface until it stops bubbling (approximately 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.

Moisture Vapour Tests
Capillary moisture in the concrete may be detrimental to the performance of certain coating systems that cannot tolerate moisture on or within the surface boundary. A moisture vapor test can be carried out, such as the plastic patch test ASTM D 4263 for example. 

Caution Note: 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 proper breathing apparatus. DO NOT mix acid with any other chemical. 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 the burns are severe (or in all cases of eye contact), contact a physician immediately. Wash hands thoroughly before handling food. (Taken from 1.3 ACID ETCHING – CONCRETE AND MASONRY – MPI Architectural Coatings Manual)


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