Because in pre-1940 construction the smoke is not only in the rooms. Open framing cavities behind plaster let smoke travel laterally inside walls during the fire, and that reservoir re-odors every cleaned surface it touches. The diagnosis is a cavity check at the worst walls; the remedy is sealing or opening and treating the cavity, priced from findings. A second surface cleaning over an untreated cavity is the one purchase this market's experience warns against.
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