Sealers for precast concrete reduce moisture penetration, protect concrete surfaces from attack by airborne chemicals, prevent surfaces from getting dirty, make cleaning easier, and prevent blotchy surface darkening that can occur when concrete surfaces get wet. With literally hundreds of sealers available, it's a good idea to look at as many products as you have time for and to ask questions about their composition, recommended uses, longevity, and application. When evaluating sealers, check manufacturers' literature for the following items: water repellency, water vapor transmission, gloss, weathering and ultraviolet stability, resistance to efflorescence, and water permanence. When applying sealers, many factors must be taken into consideration. Some of these factors are: method of application, protective gear required, temperature, ventilation, dryness of the concrete, and cleanliness of the surface to be sealed.