very basic question on paint.

My Duster was built in Southern CA and spent its life in and around Yuma AZ. I am thinking I may be the 4th owner. The guys I bought it from used heavy skim coats of bondo and then did a poor paint job. The paint came off in huge chunks and you could still smell the primer. The rear quarter panels (both) were rusted thru below the marker lights. Both sides has rust thrus just in front of the rear tires. They just filled it with bondo. Did not even cut off the ragged rusted edges.
The spare tire well was rusted away and it was filled in with bondo. The problem is the gas tank was being held up by bondo.
The reason I bring this up is you can not tell what is under there without removing the paint