Plaster that is applied directly over a previous coat will often fail to stick, unless that coat has first been keyed. Keying is the process of scratching lines into wet plaster, using a special tool called a lath scratcher, or simply the blade of a trowel. This produces a rough surface that more firmly attaches the next coat

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Note added at   7 godz. (2012-05-05 19:53:56 GMT)
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Note added at   7 godz. (2012-05-05 20:05:52 GMT)
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Surface Scabblers - keying back plaster & render