Concrete workers hold a wide range of opinions about what causes fresh concrete to blister occasionally, especially during the cooler seasons. Blistering occurs when air gets trapped in the concrete and can't break out of the seal that may be created during finishing operations or caused by a rapidly setting surface. The air collects in spots under this airtight surface seal and creates blisters. Why? One or a combination of these factors may be responsible: winds blowing over concrete surface and reducing surface moisture; a subgrade that's cooler than the concrete; a sticky mixture with excessive fines that seal the surface quickly; lean mixtures that have to be worked excessively to produce desired finishes; concrete that's finished too early, by hand or machine; and improper use of tools. Follow some simple guidelines when finishing concrete. Air entrainment reduces bleeding and increases the surface friction. This means that proper timing of the finishing operations is necessary, and the use of a magnesium hand float is required. Float the concrete twice, if time permits. If concrete sets rapidly, float at least once, and lap each machine pass about one-half the width of the machine. When machine troweling, remove the float blades and make the first pass with the trowel blades flat to the surface. Keep the trowel blades as flat as possible for following passes, and allow adequate time between passes. Then follow the final pass with hand troweling to remove any imperfections left by the machine. If a fog spray is used for the last machine pass because of high surface friction, keep the fog to a minimum, with nozzle volume not exceeding one-half a gallon per minute. Following these steps should help keep blistering well below the problem stage.