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This project must begin with a completely clean concrete surface. The overlay will chemically bond with the concrete so any paint on the concrete must be removed. Using a grinder with a diamond blade, grind in smooth circular movements until the concrete is the only thing left (image 1). Use a 3000 psi power washer to clean the newly exposed surface and get rid of dirt and debris. The goal is to remove dirt that an ordinary hose would not (image 2).
In a clean bucket, mix eight parts water to one part muriatic acid. The acid will etch the concrete, exfoliating it and opening its pores. Muriatic acid is caustic and hazardous. Follow the manufacturer's safety precautions to the letter. In a separate bucket mix eight parts of water to one part ammonia. Flood the porch with the acid solution first. Spread it with a broom so everything is covered. Once the acid stops fizzing, flood the porch with the ammonia mixture to neutralize the acid and adjust the pH. The ammonia reacts immediately, don't wait to let it take effect. Power wash the porch again to rinse excess acid and ammonia from the porch.