This is the underside right above where the exhaust is on my 91 BMW 318i. Unfortunately it is also where the transmission mount support bolts to the body. I dont think the trans support actually supports much, looking at how it is held to the body of the car, machine screws are just slid in a track with their heads held in the track, and then the bar held to that with a nut. Not much to it. Hopefully the forces are OK and the right hand rule is in my favor (this rust is on the PS of the car, BTW). My approach is this: -scrape all loose rust to leave a rough surface (done) -coat surface in Eastwood rust stopper (done) -paint surface with Eastwood rust encapsulator -lightly sand and paint with another regular paint (possibly rust oleum epoxy spraypaint because I have it) -there is a thin sheetmetal piece that goes along that same area - doesnt seem to be a heat shield, but is OE fit and causes a gap right at the point where the rust is that lets water sit - and rust form. I was thinking of masking the exhaust pipe and then spraying in the gap with rubber undercoating. It will degrade, sure, but I need to keep water out of the area. Any other ideas? Just think floor pan, thin metal sheet attached to the floor pan for some OE reason. Rust a the edge of the metal sheet on the floor pan (pictured). Need to seal that gap. or do something. Thanks!