Quality control on a decorative stucco panel
You are producing a decorative stucco panel 2 m by 1.5 m in a listed hallway. The specification calls for a three-coat lime stucco with a polished finish and a decorative moulded border. The client wants a 'smooth as glass' finish and has rejected a sample because it shows trowel marks under raking light. The sample was made with a steel trowel and polished too early. The ambient temperature is 18 degrees C and humidity is 65%. You have a wooden float, a steel trowel, a sponge, and a range of abrasives.
1. Why did the sample show trowel marks, and how would you change the technique?
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Polishing too early with a steel trowel drags the surface and traps trowel marks because the lime has not hardened enough. The finish should be built up in coats, with the surface consolidated and then polished when it has stiffened sufficiently. A wooden float can be used to open and consolidate the surface, followed by a steel trowel only when the material is ready. Working in raking light lets you see the marks as you go. The aim is a dense, closed surface without burnishing wet material.
2. How would you set a realistic acceptance standard with the client?
Read the worked answer
Show the client a sample viewed under the same raking light that will apply in the hallway, and agree what is achievable with a lime stucco. Explain that some variation is inherent in a hand-applied lime finish and that 'smooth as glass' is not a realistic standard for this material. Agree a written acceptance criterion, such as no visible trowel marks under normal lighting at a stated distance. This protects both parties and avoids disputes at handover.
3. What quality checks would you carry out on the finished panel?
Read the worked answer
Check thickness and build-up against the specification, tap test for hollow areas, and inspect under raking light for trowel marks and fairness. Check the moulded border against the profile template. Record the checks with photographs and dates. Any defect found should be rectified before the client inspects, and the records kept as evidence of quality control.