What is the basis for prohibiting hiring for a specific, defined plastering task?

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Hiring for a specific, defined task in plastering is impermissible because the materials and surfaces vary significantly. The mud/plaster (*at-teen*) differs, some being thin and some thick. Furthermore, the surfaces of the roofs differ, some being high and others low, and the walls are similarly variable. Due to this inherent uncertainty and variability, the contract must be based on a set duration of time.