Lu,
Airline metal bashers [or knockers] do the job to an approved repair scheme, work to approved drawings and do as good a job as any manufacturer. Airline development engineers holding regulatory authority design approvals have all the relevant degrees, specialise in the repair field and know how to liaise with the right people to get any extraordinary data.
In our own recent 'Ramp Rash' incident at Heathrow, the 'Big Airways' own development engineer and metal knocking team did an excellent repair entirely in-house, except for the high-blow test after completion. [They could have done that too, but it isn't economical to keep such a special test rig lying around]. So, please don't knock the airlines until you've tried us. Those infamous ugly looking 'scab patches' though fully certified and stressed, are generally temporary, lasting only until the next long maintenance visit. (I admit that cheapskate airlines often leave them on permanently, but as I said - they are fully certified)
I well remember the 'Seattle Big Jets' AOG team who came to do a Service Bulletin on our Thrust Reversers and lived up to their name - they finished their own job but left us with a real 'AOG' and a poor impression - and it was after all, a 'Seattle Big Jets' team that screwed up the JAL repair...
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Through difficulties to the cinema