struck off flush or slightly raked. Old walls softened anddisintegrated by weather action, surfaces that cannot becleaned thoroughly, such as painted brickwork, and allmasonry chimneys should be covered with galvanizedmetal reinforcement before applying the stucco. Whenmasonry surfaces are not rough enough to provide goodmechanical key, one or more of the following actionsmay be taken:Old cast-in-place concrete or other masonry maybe roughened with bush hammers or othersuitable hand tools. Roughen at least 70 percentof the surface with the hammer marks uniformlydistributed. Wash the roughened surface free ofchips and dust. Let the wall dry thoroughly.Concrete surfaces may be roughened with anacid wash. Use a solution of 1 part muriatic acidto 6 parts water. Note: Add muriatic acid to thewater; never add water to the acid. First, wetthe wall so the acid will act on the surface only.More than one application may be necessary.After the acid treatment, wash the wallthoroughly to remove all acid. Allow the washedwall to dry thoroughly.CAUTIONWhen your crew members are usingmuriatic acid, make sure they weargoggles, rubber gloves, and otherprotective clothing and equipment.You can quickly rough masonry surfaces using apower-driven roughing machine (such as thatshown in figure 7-21) equipped with a cylindricalcage fitted with a series of hardened steel cutters.The cutters should be mounted to provide aflailing action that results in a scored pattern.After roughing, wash the wall clean of all chipsand dust and let it dry.Suction is absolutely necessary to attain a properbond of stucco on concrete and masonry surfaces. It isalso necessary in first and second coats so the followingcoats bond properly. Uniform suction also helps obtaina uniform color. If one part of the wall draws moremoisture from the stucco than another, the finish coatmay be spotty. Obtain uniform suction by dampeningthe wall evenly, but not soaking, before applying thestucco. The same applies to the scratch and brown coats.If the surface becomes dry in spots, dampen those areasagain to restore suction. Use a fog spray for dampening.Figure 7-21.—Power-driven roughing machine.When the masonry surface is not rough enough toensure an adequate bond for a trowel-applied scratchcoat, use the dash method. Acid-treated surfaces usuallyrequire a dashed scratch coat. Dashing on the scratchcoat aids in getting a good bond by excluding air thatmight get trapped behind a trowel-applied coat. Applythe dash coat with a fiber brush or whisk broom, usinga strong whipping motion at right angles to the wall. Acement gun or other machine that can apply the dashcoat with considerable force also produces a suitablebond. Keep the dash coat damp for at least 2 daysimmediately following its application and then allow itto dry.Protect the finish coat against exposure to sun andwind for at least 6 days after application. During thistime, keep the stucco moist by frequent fog-spraying.MixingMixing procedures for stucco are similar to thosefor plaster. Three things you need to consider beforemixing begins are the type of material you are going touse, the backing to which the material will be applied,and the method used to mix the material (hand ormachine). As with plaster, addition of too much of oneraw ingredient or the deletion of a raw material givesyou a bad mix. Prevent this by allowing only therequired amount of ingredients in the specified mix.ApplyingStucco can be applied by hand or machine. Machineapplication allows application of material over a largearea without joinings (joinings are a problem for7-18
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