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Aliphatic urethanes are similar to epoxy sealers. They are both surface level concrete sealers that feature large particles of sealant that, rather than penetrate past the tiny surface pores natural to all concrete, will remain on the surface of the concrete to be spread out evenly during the application process. Once so spread out, the aliphatic urethane (like the epoxy sealer) will feature a strong adhesive bond to the concrete substrate, protecting the concrete underneath from abrasion, heat, water, chemical spills, and similar threats. However, aliphatic urethanes are significantly stronger than epoxies, featuring a greater level of resistance to the aforementioned threats. This makes aliphatic urethanes the preferred choice for industrial warehouses that feature heavy machinery and frequent chemical spills, as well as airports and large retail stores that have heavy foot traffic and frequent spills. Aliphatic urethanes and epoxy concrete sealers do not actually have to be mutually exclusive products, either. In fact, many people frequently choose to apply an epoxy base layer to the concrete substrate before adding a layer of aliphatic urethane over that epoxy layer. The aliphatic urethane will actually bond just as well to the pre-existing epoxy as it will to the concrete itself, making it the perfect choice for a topcoat. Of course, aliphatic urethanes are not the perfect choice for every type of concrete project. Sometimes you will need a penetrating sealer, like a concrete densifier, to achieve the desired results. Concrete densifiers are different from aliphatic urethanes primarily because they feature small particles of sealant that will penetrate past the surface pores natural to all concrete, passing through the extensive network of microscopic pores and hairline cracks to react chemically within the concrete. These chemical reactions actually enhance the very nature of the concrete, greatly increasing its strength, longevity, and structural integrity. Concrete densifiers are preferred to aliphatic urethanes for smaller projects, such as for homes or buildings, rather than large retail stores, airports, or warehouses. They are often waterproofers (if you find a silicate/siliconate variety), and some even create vapor barriers that make them the most effective type of concrete sealer against potential water damage. They feature many of the same benefits as aliphatic urethanes, though they are not as strong and will not withstand as harsh of conditions. Our Take: Some excellent aliphatic urethane choices we recommend are Superior Industries Aliphatic Urethane or Urethane 645. As far as concrete densifiers we recommend Lion Hard, Lithi-Tek 4500 and Formula One