The corroded seam, about 15 inches long, that I am trying to fix is on the outside edge of the passenger rear floorboard. I have seen this same seam damaged in about 4 or 5 W123s.( So please forget abou the idea of cutting one from another car!)
The front of the seam is only one inch behind where the front passenger side seatbelt bolts to. It runs straight back and the rear of the seam is the just prior to the hump where the bench seat of the rear seat rests.
Inspection of the damaged area from underneath reveals that the steel in this area was stamped and has several wells and raised points and is not a simple flat plain surface. Also it was covered with undercoating and really needs to be grinded!
Inspection of the damaged area from the top shows that two layers of steel are damaged. The first layer is the floorboard I just mentioned and the second layer is an inner area that forms the door edge hump.
I have heard that floorboards should always be repaired by welding from the bottom. Should I start grinding the area from underneath meanwhile?
I have at least a month before I finish reading my Haynes Welding manual and practice with my equipment, but i would like to start the surface prep now.