Reached an important stage today! The sill on the left is welded shut too. Started on time as usual. The inner sill had to get a new patch at the front, so I first sawed out the suspect piece.
Made a new piece of metal and welded in and it immediately looks a lot better.
The other side after welding
Before you can spot weld it must be 100% CLEAN:
And don't forget to test-fit:
Making a step in the old sheet metal
Neatly spot welded and the long seams welded nicely without deformation.