Mark, that is a sound deadner Ford shot into the 1/4's by wheel well.Spent the entire day in the garage. Started with PB Blastering the seat belt bolt that was cut off for removal(amazing what having the right tool and liquid assistance can do). Then moved on to removing more undercoating (I don't know if it was Stretch Ziebart or Stretch Ford guy but he sure did reach deep into the rear quarters).Then sizing up floor panels while I wait for my kids Christmas gifts of driverside complete floor and rear fraim rail and replacement rear frames. Man I've got some great kids!!
You should put something back in after you kill the rust.