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Yes, you can seal old or existing concrete, but proper preparation is crucial. The concrete surface must be clean, dry, and free of any incompatible old sealant. To check if an old sealer is present, drip a few drops of water onto the surface; if the water is absorbed in under five minutes, it's ready to be resealed. If you plan to use the same type of sealer that was previously applied, you may not need to remove the old layer. However, if you are using a different type of sealer, the old one must be completely removed. Before applying the new sealer, thoroughly clean the concrete and repair or fill any cracks to ensure a smooth, well-prepared surface.

No. You cannot use asphalt products on concrete driveways. To seal a concrete driveway, use acrylic, epoxy, polyurethane, silane, siloxane, and silicate sealers or deep penetration sealers made specifically for concrete. Meanwhile, asphalt sealers contain refined coal tar (asphalt emulsion), water, sand, clay, polymers, and additives and are incompatible with concrete’s surface texture and higher density.

If your concrete stains are green in color, it indicates the presence of mold or algae—which you should address right away. If left unattended, mold will spread to other areas throughout the home. However, be careful not to disturb mold on indoor concrete surfaces and work with a local mold remediation specialist for specialized care. Once free from mold and algae, seal your concrete to eliminate pores and gaps.

Concrete sealers will help your garage floors resist water, but they will not make the floor waterproof. You will need to purchase a specific waterproof floor paint for concrete and apply several coats over several days if you want your garage floor to be truly impermeable. While the process is similar to sealing the floors, the materials are different, and the need for multiple coats will extend the process and drying time.