PLASTER special subjects They usually form a network of thin and shallow cracks that do not extend to the entire depth of the plaster. Cracks of this type occur within a few hours of plastering and are not visible unless moisture and dust settle on them. These surface cracks are of minor importance, do not open or close with time, and can be covered with a suitable quality (elastomeric) paint. The composition of ‘structural cracks’ usually involves deeper cracks (sometimes through the entire thickness of the plaster mass). These cracks usually occur when using a plaster mixture with a high cement content that is allowed to dry very quickly. Cracks found when the plaster is still green (fresh) are sealed with the plastering float and worked over, but usually, they reappear later. The solution for their proper sealing is by using special, resinous sealants that contain ceramic and/or glass microspheres or quartz aggregates. Otherwise, they are filled with the use of suitable sealants or paints (for capillary cracks). Dehydration or shrinkage cracks occur when a large amount of water (moisture) evaporates from the plaster mass within the first hours after application. The same goes for horizontal cracks that form at the corners and between door and window frames. Plasters usually form cracks throughout their lives. In any case, these cracks must be repaired and the surfaces must be painted with the appropriate paints, ideally with elastomeric type paints. In cases of multiple and extensive cracks in the plaster, and after the spot repair with the appropriate repair products, it is highly recommended to coat the entire surface with resinous mortars to a thickness of about 3mm, with the simultaneous encasement of an alkalineresistant fiberglass mesh. Once dry, coat the entire surface with flexible plasters in paste or cementitious form. SURFACE CRACKS DEEPER CRACKS Repair & Renovation Guide 65 THE MOST COMMON DAMAGE OF PLASTER The damages that are usually observed in some plastered surface can be categorized as follows: Surface cracks (hairline cracks) Bursting Structural cracks (deep in the mass of plaster) Peeling Detachments Patch marks Lack of hardness
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