You can also replace panels just because the originals have too much bondo.

My example is my passenger door. It obviously got hit hard sometime in the past. The repair shop drilled a line of holes from the front to the back roughly along the center line that I could see from the inside (bondo snakes). I sanded the entire door down at one point to see how much bondo there really was and saw that it was truely filler from end to end.

So I had a shop put a new outer skin on.