Problems with coatings often rank No. 1 with decorative concrete contractors. They arise when sealed surfaces become white or frosty, scale off, or blush. These appearances usually result from coatings losing their bond to the surface of the concrete. Sometimes the loss of bond can be in very small areas causing clear coatings to look frosty, because light is diffused by the air between the concrete and the film. The condition can be further complicated by the development of efflorescence on the concrete surface.
The difference between sealers and coatings isn't well defined except sealers also can be penetrating products that are impossible to completely remove. For simplicity, the word coatings will be used to refer to products that build up thickness on the surface of concrete.
The two most common reasons a bond fails are inadequate surface preparation or excessive moisture content in the concrete.
When acrylic coatings develop frosty appearing surfaces, the first thing many contractors do is scrub a solvent such as xylene (xylol) into the sealer to dissolve the polymer film, drive it deeper into the concrete, and reattach it to the concrete. If that doesn't solve the problem, then removing it is the only option.
Coatings can be removed mechanically by sandblasting, beadblasting, diamond grinding, or sanding. However there is risk of damaging or altering the concrete surface, causing it to reflect through a new coating.
Stan Stratton, the director of technology development for L.M. Scofield, Douglasville, Ga., says coatings have many different chemistries and some even crosslink to increase the durability and chemical resistance of the in-place film. Therefore, coating removers must be aggressive chemicals that destroy the integrity of the coating film by dissolving it or causing it to swell and weaken the bond to facilitate removal. Chemical strippers or removal products are the most commonly used for coating removal.
In the past, most strippers or removers contained methylene chloride and were effective on a wide range of polymer coatings. But Stratton says they have been determined to be high-level environmental pollutants even though they are still VOC exempt. So to avoid this problem, formulators are trying to develop better coating removal chemistry.
Products for removing coatings
Below there is a list of the common generic coatings and a list of generic products that can remove them. Read product labels and instructions to determine which products to use, and always try a test strip first.
Two strippers you may not be familiar with include special alcohols or blends of these alcohols with hydrogen peroxide. They have been in use for industrial purposes for more than 20 years but recently are being used by contractors for coatings on concrete.
Joe Garceau, an owner of Butterfield Color, Aurora, Ill., says they became interested in alcohol-based stripper technology because it's effective and environmentally friendly. Unlike solvent products, alcohol strippers don't soften coatings, turning them into a gooey mess that must be scraped and wiped off surfaces. They cause coatings to lose their bond with concrete, leaving residue that can be pressure washed off.
Chuck Knight, a sales manager for Sto Corp, Atlanta, says their product incorporates alcohols and hydrogen peroxides that pass through coatings, debonding them from the substrate. He adds that the dwell time (the time required for the stripper to act) is between four to six hours depending on the material and its thickness.