HELP ME UNDERSTAND SKIM COATING

My old '67 fastback was like that on the right side, but it was due to incredibly poor body work. The right quarter had been damaged in an accident and then a monkey with a hammer stretched things farther. Lots of mud on the quarter which now did not match the near perfect door. So, mud the door, which now didn't match the fender that was straight at the door (it had issues up front), so mud the fender. We were able to shrink the metal and get it close with much less Bondo. The cowl was also damaged on that car with no attempt to get it back to straight. Removed the AC box and was able to get it really close by just pushing it out...