Question: What’s the best and fastest way to prep a concrete surface before applying a decorative overlay?
Answer: First, it’s important to note that if you don’t do a sufficient job of preparing the substrate, all of your painstaking artistry could be a waste of time and effort. I would bet that 90% of overlay delaminations and failures are due to inadequate surface preparation.
This critical step involves more than simply cleaning the substrate and removing existing coatings. Obtaining the proper concrete surface profile (CSP) is equally important. Overlays bond best to surfaces with a rough, sandpaper-like finish. The thicker the overlay or topping, the more aggressive the profile should be.
There are several ways to profile the concrete surface. Follow the overlay manufacturer’s recommendations for the best method to use, since requirements may vary depending on the properties of the system you’re applying.
Mechanical profiling
Mechanical abrasion is the best way to remove most contaminants and unsound concrete. It breaks up existing sealers, coatings, or adhesives, and also lightly pulverizes the surface, leaving a roughened profile for overlays and toppings to grab onto.
Shotblasting. One of the most cost-effective methods of removing contaminants from a large area and for prepping substrates for self-leveling or polymer overlays, shotblasters use centrifugal force to propel steel shot at high velocity onto the surface. The big advantage of shotblasting when compared to other mechanical methods is it produces very little airborne dust or debris. The process is confined in an enclosed blast chamber that recovers and separates the dust and reusable shot.
The International Concrete Repair Institute recommends a maximum removal depth of ¼ inch per pass. That much may not be necessary for overlay systems unless you’re stripping away thick coatings or adhesives.
Dustless grinding. Many of today’s dustless grinders can be fitted with diamond-segmented abrasives of various grit levels ranging from fine to coarse, depending on how aggressive the profiling needs to be.
Scarifying. Scarifier attachments can be affixed to grinders to remove thick coatings and mastics. Many contractors also use grinders with finer-grit abrasives to give traditional concrete surfaces and some overlays a high-polish luster. A dust containment system vacuums virtually all of the dust generated during grinding.
Acid etching
This method typically involves applying a solution of water and muriatic or citric acid to concrete (via low-pressure sprayer or plastic sprinkling can) to chemically remove the cement paste at the surface and lightly expose the fine aggregate. The substrate should be damp to prevent burn marks where the acid first hits the surface, and the acid should be scrubbed into the surface with a stiff-bristle broom.
Acid etching may be unsuitable for certain applications and environments. Because it produces a very light profile similar to fine sandpaper, it’s generally suitable only for skim coats or microtoppings less than 10 mils thick. You also must take great care to neutralize the acid and remove residual solution from the surface. Not only does the acid residue act as a bond-breaker, but it can also penetrate into porous concrete and chemically react with the cement, affecting the integrity of the concrete surface.
I use a cleaning solution of one quart of ammonia to five gallons of water. A solution of baking soda and water is also effective. Remove the remaining residue with an industrial wet/dry vac or squeegee vac. I recommend a final power washing to ensure a clean substrate.
Cleaning up the spent acid is often a messy process requiring a lot of rinsing, scrubbing, and copious amounts of water. Also, during the acid wash, contaminants lifted from the surface may contain toxic materials such as tile mastics, lead-based paints, or asbestos. Be sure to check your local environmental regulations regarding containment and disposal of the acid and rinse water.
Worker safety is also a big concern. Applicators must wear the necessary protective gear to prevent eye and skin burns. The acid could also react with and corrode any metal components it comes in contact with, such as electronic equipment and machinery, so these elements must be protected as well.
Surface preparation can be a tedious, time-consuming, and messy task. But don’t be tempted to take short cuts. Only by doing the job properly and thoroughly can you ensure long-term bonding of the overlay—and a satisfied customer.
Bob Harris, founder of the Decorative Concrete Institute, Temple, Ga., and senior decorative concrete consultant for Structural Services Inc., has more than 25 years of experience in the construction industry. He conducts seminars in architectural and decorative concrete worldwide, is involved with numerous associations, and is a popular speaker at World of Concrete and other events. The information in this article is based on Bob Harris’ Guide to Concrete Overlays & Toppings, which now comes with a DVD that provides step-by-step instructions for rejuvenating floors and exterior flatwork. Visit www.decorativeconcreteinstitute.com.
