HELP: C-Pillar Base Rust Repair

[arrestedthread]

Area depicted in photo is where the C-pillar base meets the quarter panel shoulder (rear driver side) on an E30 Touring.

Is it possible to access the other side of this rust hole? I'd like to clean this area out from the other side and weld the hole with the aid of a copper backing plate.

Any tips on access would be greatly appreciated.

[GasolineJess]

Short answer: usually no direct path to the back of that spot, so you make one. It's double-walled on the Touring, which is exactly why it rusts there. First step: pull the rear cargo side trim and see what you can reach from inside. If the back is closed steel (usually is), don't fight it. Cut a small access window in the most concealed part of the inner structure, somewhere that ends up hidden under trim when the car goes back together. Do the repair through the window, then weld it shut and seam-seal it.

The copper backing plate is the right tool for thin sheet. Cut back to bright, solid metal first, and keep opening until it's firm. If the steel goes papery after wire-brushing, bridging it is how the same hole comes back in two winters. Tack small and move around so you don't build heat.

Find the water path before you close anything up: hose the roof gutter and tailgate area down and watch where it tracks. That corner on a Touring usually gets fed from the hatch area or the rear glass, not just wet air. Fix the source, epoxy-prime the bare metal, seam-seal the joints, and cavity-wax the inside through your access window before you weld it shut.

(Longer version of this is over on the body board, same title.) Post a shot with the trim off and I'll point at where I'd open it. - Jess (I do body and paint for a living)