Auto body repair & detailing: Questions to aks to spot a good car body repairer., exact science, good car

Question
I took the car to a renowned local mechanic to patch a side of the bumper of my almost new car.  It was slightly scratched, and it had no body structure damage.  The result after the patching was: the paint was ok, but the scratch beneath was still showing, and infact they are going to re-do it as the job was said to not have been done well.  For future reference, what questions should I ask to know how to choose a good mechanic?  What do you suspect went wrong in this case with the bumper scratch still showing beneath a patched paint?    And do one needs to disconnect battery to be able to perfom patching on bumpers?  Many thanks for your help!!  Much appreciated.


Answer
Hello Racheal
OK , I'm glad they will re-do it for you.
It sounds like the repair procedure was not strong enough to withstand the solvents in the paint , which is why the top coats have sunk into the substrates.
Referrals are one way  to qualify a good shop, but it isn't an exact science.
We only find it necessary to disconnect a battery if we are dong welding.
Hope all this is helpful
Dan