@kandgmitchell, and @saveasteading point to a generalisable problem
inaccurate or poor workmanship is commonly excused by reference to the follow-on trades : or preceding trades
and that process continues until chippies can justifiably say that they compensate for everyone else's errors of commission or omission.
Added to that, there's a pernicious tendency to attribute fault to a customer who (as in this case) has the temerity to draw attention to a genuine problem.
@kandgmitchell 's formula - reference to standards required by others - is a superb way of professionalising matters.
Works well across all professions - apart from undertakers.