Pressure-washing the engine bay at home
Water driven past connector seals into coils, the alternator or fuse boxes causes faults that surface days later as corrosion develops, which is why the damage rarely gets connected to the wash. Bays need degreaser, brushes, covered electronics and low pressure or steam — a job for someone who has done it a few hundred times.
Judging vendors on the handover shine
Fresh dressing and a wet-look finish flatter any job for 48 hours. The real quality indicators are process questions asked before booking — wash sequence, decontamination steps, depth-gauge use — and how the paint looks under an inspection lamp at handover. Comparing two or three scoped quotes rather than one glossy promise is the cheapest quality control available.
Polishing too often
Every machine polish removes clearcoat, and clearcoat is a finite budget of roughly 40–60 microns for the car's whole life. Cars that get 'a quick buff' at every service end up with thin, UV-vulnerable paint within a few years. Protect more, polish less: correction should be an occasional reset, not routine maintenance.