Buying a ceramic coating without paint correction underneath
A coating is transparent and permanent enough to be annoying: it seals in every swirl and etch mark beneath it. The suspiciously cheap coating deals almost always skip decontamination and polishing, which is precisely where the labour cost lives. Pay for the prep or skip the coating.
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.
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.