I reckon the last 3 times a valance has been replaced the old one has been chopped off and the lip that is spot welded on has been left in situ. The new valances have then been spot welded in place through the previous chopped off lips.
This resulted in between 1, 2 or 3 layers of rusty metal to get rid of. The layers varied depending on the level of decay that rust had caused along the lip of the rear panel.
Workin from under neath the car I used a grinding disk to get rid of all the old layers of crust.
This pic shows the finished right half and the un-finished the left half.