Repainting a school corridor with a damp patch and chalking distemper
A senior secondary school in Patiala wants its 15-year-old corridor repainted during a 10-day autumn break. The work is 220 m² of wall and 60 m² of ceiling, cement plaster, previously finished in oil-bound distemper. Your inspection finds hairline cracks up to 0.3 mm, a 3 m² patch of fluffy white efflorescence about 300 mm above the skirting beside a leaking rainwater downpipe, chalky powder that comes off on your hand wherever the distemper is thin, and green algae on the north wall. A moisture meter reads 19% at the downpipe patch and 9–11% elsewhere; ambient RH is 78% and the break has had two rain days. The head teacher's sanctioned estimate reads “one coat primer, two coats acrylic emulsion, no putty”, and she wants the corridor usable on day 9 for a state inspection. Your gang is three painters, one of whom has never used a moisture meter.
1. Do you start work as specified? Decide, and set out what you would tell the head teacher about the estimate and the day-9 deadline.
Read the worked answer
No - not as specified. Three findings each individually stop the specification:
- 19% moisture at the downpipe patch. Most acrylic emulsion datasheets require the substrate below roughly 12–15% when measured with a moisture meter; 19% means the wall is still wet. The cause is the leaking downpipe, which is not a painting item at all. Painting over it traps water and guarantees blistering within one monsoon.
- Efflorescence is soluble salts carried to the surface by that same water. It is a symptom, not a stain; wire-brushing it off without curing the leak only moves it.
- Chalking oil-bound distemper is a friable layer with almost no cohesive strength. Emulsion applied over it bonds to powder, not plaster, and will flake in sheets.
What I would say to the head teacher: “The estimate covers paint, not preparation. Two of the problems - the leaking pipe and the wet wall - are civil works; until the pipe is repaired and the wall reads under 15%, any paint will fail and you will pay twice. Of the ten break days, we lose two to rain; the downpipe repair and drying need four to five. So I propose: dry areas (about 90%) prepared and painted in days 1–7, the damp patch left open for repair and painted in a second visit. I will give you a written variation for the putty and the surface preparation before anyone lifts a brush.”
Get the variation signed. Starting “because the head teacher said so” transfers all the failure risk to you, and distemper-over-emulsion failures are visible in the first season.
2. Give the full preparation and coating sequence for the corridor, from the downpipe patch to the final coat, naming product types and the checks between stages.
Read the worked answer
Stage 1 – cure the source. Repair/replace the leaking downpipe and its joint; redirect the outlet away from the wall. No coating work at the patch until this is done and the wall has dried.
Stage 2 – strip and clean. Scrape and wire-brush the efflorescence back to sound plaster; wash the area with clean water and allow to dry. On the north wall, apply a proprietary fungicidal/algicidal wash, leave 4–6 hours, then rinse and dry. Remove all chalky distemper by scraping plus washing with a distemper remover, using stiff brushes; a pressure wash is faster but needs 2–3 dry days afterwards.
Stage 3 – cracks and filling. Open hairline cracks to a shallow V, dust out, fill with polymer-modified mortar or acrylic crack filler, then skim with wall putty, sand with 180 then 220 grit, and dust off.
Stage 4 – moisture check. Take three readings across the corridor and log date, location and value; proceed only below the manufacturer's limit (typically ≤ 12–15%). This is where the untrained painter gets shown the meter and does one reading under supervision.
Stage 5 – priming. Alkali-resisting primer on all patches, bare plaster and filled areas, at datasheet thinning, full coat.
Stage 6 – putty. Two coats of white cement-based wall putty, sanding between coats, then a primer coat over the putty.
Stage 7 – finish. Two coats of acrylic emulsion, first coat thinned per datasheet, recoat after 4–6 hours at 25–30 °C; ceilings in the same sequence without putty.
Between-stage checks: adhesion scratch test on a trial patch, moisture re-read after rain, and a signed area-wise handover before the inspection.
3. The head teacher asks what happens if you simply paint over everything to save four days. Give the failure mechanisms, the likely time to failure, and the minimum you would not compromise on.
Read the worked answer
Paint-over failure mechanisms
- Moisture blistering: water in the plaster turns to vapour under solar gain, exerts pressure behind a low-permeability acrylic film, and lifts it in domes; the film ruptures and the plaster is exposed. Typical onset: one monsoon, often within 3–6 months.
- Efflorescence recurrence: salts crystallise in and under the film and push it off. Because the source (the leak) remains, the patch grows rather than heals.
- Delamination over distemper: emulsion bonds to a chalky, weakly bonded layer; the weak link fails at the distemper/plaster interface and whole sheets release - often 6–12 months, and in one visible sheet while the school is in session.
- Algal regrowth: spores survive under the film and reappear through it in the next damp season; no coating resists that without the wash step.
Minimum I would not compromise on (in order): (1) the downpipe repair and a moisture reading under the datasheet limit; (2) complete removal of the chalking distemper; (3) alkali-resisting primer on patches; (4) two finish coats on the walls.
What can give way to recover the four days: putty only where the plaster is uneven, chalk-line straightness rather than full skim, ceiling in one coat over a sound substrate if written into the variation, and phasing the damp patch to a second visit. Those cut time without creating a guaranteed callback. State each substitution in the variation so the school knows exactly what it is buying.