Complex sentence: The old timers considered the use of plastics "as poor craftsmanship," hence not only did the old times consider the use of plastic as insulting, but also the indication that the bodyman who made the repairs a novice.
Split into simpler sentences: The old timers considered the use of "plastics" as poor craftsmanship. Hence, not only did the old timers consider the use of plastic as insulting but also an indication that the bodyman who made the repairs was a novice.