Was bored this afternoon so did some research on how that rear section was put together.
Link>
Side panel. Fits: Volkswagen (VW) | Jim Ellis VW Parts
Notice parts marked # 7 2 pieces I think were welded together and then #13 the rocker panel section was added and the whole works spot welded under the body in one piece.
Anyway all I have is #13 note that the number varies from one model NB to another.
So I got out my trusty Harbor Freight air chisel with the air pressure turned down and started to separate the top seam. Can't tell if its the rust or is it just not spot welded that well. But its not all that tight. When I move further back, the seam goes underneath the side panel. I may, haven't decided yet, just cut that part out from the rocker panel with a cut off disk. I will do more tomorrow. Not looking forward to removing the inter fenders.
Oh the white stuff, spray foam. The reason this is rusted, sometime in the past it was damaged real bad, and it had lots and lots of Bondo, and not sealed with undercoating.