The soot trail behind the exhaust. Completely gone, without harming the paint.
Every turbine rotorcraft carries it: the grey-black veil running from the exhaust across the cowling and down the tail boom. It is the first thing an owner sees when walking around the machine, and the last thing a wash crew ever actually removes. The reason is simple: turbine exhaust soot is not dirt sitting on the paint but a thin, baked-on film. Pressure and force do not help; they only attack the paint.
We work in steps rather than by force: soaking with an aviation-approved cleaner, mechanical work only as far as necessary and only with soft tools, then inspection under raking light and, where the paint allows it, gentle machine polishing of the affected zone. Before every machine step we measure paint thickness. Anything that cannot be removed without taking off paint stays and is documented as a finding, not sold as a success.
You receive the zone photographed before and after from the same angle, with timestamps, together with the batch numbers of the products used. With regular care the trail returns far more slowly, because a smooth, sealed surface gives soot less to hold on to than a dulled one.