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The surface preparation for concrete overlay is the most important step if you want it to last. If possible, you need to get down to the bare concrete, especially on exterior surfaces. This means removing any paints, sealers, glues and anything else that may inhibit a strong bond between the concrete and the overlay.
The next step is acid etching. Acid etching serves two purposes in getting the concrete ready for an overlay. It opens the pores of the surface allowing the overlay material to penetrate and create a mechanical bond. Acid etching is also one step in setting the pH level of the concrete.
Dilute muriatic acid to 5 parts water:1 part acid. Using a plastic sprayer, completely soak the surface and let it sit for several minutes before rinsing. Be sure to use caution and protection when working with acid.
After rinsing the acid and residue, the next step is neutralizing the surface. Dilute ammonia to 5 parts water:1 part ammonia. Then spray on and completely soak the surface. The process of acid etching and neutralizing will adjust the pH of the surface to accomodate a chemical bond between the concrete and the overlay.