Ask yourself - why would someone go to the trouble of applying filler and painting it, without spending another 15 minutes on a half-decent sanding job.

Grab yourself a fridge magnet - if it sticks, it's metal. If not, it's filler under the colour.

How long since your last visit to your mechanic did you notice the issue? Because it would have stuck out like dogs balls to me. Unless your last visit to them was in the last few days to a week, I'd be questioning why you didn't notice it beforehand - if they actually did the damage.

If it were me, I would not be going and accusing someone of something without having damned good proof. IMO, this is not damned good proof.

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Cheers,

Anthony