How would you go about repairing this rust?

[Colt601]

91 318is... recently purchased and spotted this while dropping the passenger side control arm. It's right above the exhaust and o2 sensor.

Not an easy spot to get at for repair

[tomstin]

Ahhh, is that under the heat shield for the cat? Been there! Mine had rusted through and needed to cut out pieces of the floor and weld new pieces in.

First question, is it through yet or surface? You will need to remove the heat shield and see how bad.

What I ended up doing after the repair, I installed stainless bolts through the floor, double nut the under side, fender washer, heat shield, fender washer and nut. My theory is leaving an air gap between the heat shield and body.

I used POR15 on both sides and seam sealer on the seams.