Author Topic: Found Rust In Rear, Under Battery Tray...Fix-It Tips? (Read 4228 times)

bmwman91 · « **on:** August 25, 2009, 06:39:20 PM »

While my car was on a lift getting a new catalytic converter today, I found a suspicious looking patch of rubberized paint. Just under the rear apron on the passenger side, the paint was rotten & there was some rust. I picked at it with a screwdriver, & it went all the way through! So, I have a 1"x2" patch of shitty metal back there, more or less under the battery tray. What is the best way to fix this? I was thinking of getting a steel brush under there & knocking off as much crap as possible, neatly cutting the edges & welding in some steel sheet & applying truck bed-liner.

However, that seems totally overkill. Would bondo/JB Weld be OK for filling the small parts that are actually gone (1"x2" is rusty, but only a very small portion is actually rotten through)? Then how about applying some heavy PVC underbody coating to both sides of this?

Thanks!

Ramblin MAn · « **Reply #1 on:** August 25, 2009, 08:42:21 PM »

Bondo absorbs water like a sponge. If it gets any moisture to it at all it will fail and you can expect it to get plenty there.

I'd cut the bad spot out and "glue" a new piece in with some silicone, let it dry a couple days and then use the undercoating on it.

bmwman91 · « **Reply #2 on:** August 25, 2009, 11:38:11 PM »

Interesting, thanks for the info on Bondo. I have a MIG unit at my parents' place, and I have gotten pretty good with it. Maybe I will yank the battery & trunk carpet & make a more permanent solution. I'll post pics.

e30trash · « **Reply #3 on:** September 09, 2009, 10:29:22 PM »

I just went through the same issue on mine. Seems that there is a little pocket where the tow hook is that traps water. Over time the water seeps through two panels (rear valence and battery box) which then rots from the inside out. I spent a couple of days trying to figure out the best way to prevent this from happening again.
So what I came up with is to cut out said pocket and weld in a new patch panel that deletes the "pocket", seams seal and under coat.

I have not taken pics since I finished it up but I will take some pics and post em tomorrow.

monty23psk · « **Reply #4 on:** September 09, 2009, 11:40:55 PM »

Look under my profile for some work I did regarding the battery acid vent. I guess because of the acid, the rust is worst in this area.